Correction from previous (now hidden) comment - sorry, pasted wrong log.

Added the arguments requested by Konrad. If there are other kernels you'd like us to try to pinpoint which kernel introduced the issue, please let me know.

(XEN) Xen version 4.1.2 (Ubuntu 4.1.2-2ubuntu2.5) (<email address hidden>) (gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) ) Tue Jan  8 14:07:12 UTC 2013
(XEN) Console output is synchronous.
(XEN) Bootloader: GRUB 1.99-21ubuntu3.7
(XEN) Command line: placeholder loglvl=all guest_loglvl=all com1=115200,8n1 console=com1,vga noreboot sync_console
(XEN) Video information:
(XEN)  VGA is text mode 80x25, font 8x16
(XEN)  VBE/DDC methods: V2; EDID transfer time: 1 seconds
(XEN) Disc information:
(XEN)  Found 2 MBR signatures
(XEN)  Found 2 EDD information structures
(XEN) WARNING: MTRRs do not cover all of memory.
(XEN) Truncating RAM from 9109504kB to 9043968kB
(XEN) Xen-e820 RAM map:
(XEN)  0000000000000000 - 000000000008f000 (usable)
(XEN)  000000000008f000 - 00000000000a0000 (reserved)
(XEN)  00000000000e0000 - 0000000000100000 (reserved)
(XEN)  0000000000100000 - 00000000ce674000 (usable)
(XEN)  00000000ce674000 - 00000000ce6cc000 (ACPI NVS)
(XEN)  00000000ce6cc000 - 00000000cf5fb000 (usable)
(XEN)  00000000cf5fb000 - 00000000cf608000 (reserved)
(XEN)  00000000cf608000 - 00000000cf6a6000 (usable)
(XEN)  00000000cf6a6000 - 00000000cf6ab000 (ACPI data)
(XEN)  00000000cf6ab000 - 00000000cf6f2000 (ACPI NVS)
(XEN)  00000000cf6f2000 - 00000000cf6f3000 (usable)
(XEN)  00000000cf6f3000 - 00000000cf6ff000 (ACPI data)
(XEN)  00000000cf6ff000 - 00000000cf700000 (usable)
(XEN)  00000000cf700000 - 00000000d0000000 (reserved)
(XEN)  00000000fff00000 - 0000000100000000 (reserved)
(XEN)  0000000100000000 - 0000000228000000 (usable)
(XEN)  0000000228000000 - 000000022c000000 (unusable)
(XEN) ACPI: RSDP 000FE020, 0014 (r0 INTEL )
(XEN) ACPI: RSDT CF6FD038, 004C (r1 INTEL  DG965SS       685       1000013)
(XEN) ACPI: FACP CF6FC000, 0074 (r1 INTEL  DG965SS       685 MSFT  1000013)
(XEN) ACPI: DSDT CF6F7000, 40E9 (r1 INTEL  DG965SS       685 MSFT  1000013)
(XEN) ACPI: FACS CF6AB000, 0040
(XEN) ACPI: APIC CF6F6000, 0078 (r1 INTEL  DG965SS       685 MSFT  1000013)
(XEN) ACPI: WDDT CF6F5000, 0040 (r1 INTEL  DG965SS       685 MSFT  1000013)
(XEN) ACPI: MCFG CF6F4000, 003C (r1 INTEL  DG965SS       685 MSFT  1000013)
(XEN) ACPI: ASF! CF6F3000, 00A6 (r32 INTEL  DG965SS       685 MSFT  1000013)
(XEN) ACPI: SSDT CF6AA000, 01BC (r1 INTEL     CpuPm      685 MSFT  1000013)
(XEN) ACPI: SSDT CF6A9000, 0175 (r1 INTEL   Cpu0Ist      685 MSFT  1000013)
(XEN) ACPI: SSDT CF6A8000, 0175 (r1 INTEL   Cpu1Ist      685 MSFT  1000013)
(XEN) ACPI: SSDT CF6A7000, 0175 (r1 INTEL   Cpu2Ist      685 MSFT  1000013)
(XEN) ACPI: SSDT CF6A6000, 0175 (r1 INTEL   Cpu3Ist      685 MSFT  1000013)
(XEN) System RAM: 8053MB (8247112kB)
(XEN) No NUMA configuration found
(XEN) Faking a node at 0000000000000000-0000000228000000
(XEN) Domain heap initialised
(XEN) found SMP MP-table at 000fe200
(XEN) DMI 2.4 present.
(XEN) Using APIC driver default
(XEN) ACPI: PM-Timer IO Port: 0x408
(XEN) ACPI: ACPI SLEEP INFO: pm1x_cnt[404,0], pm1x_evt[400,0]
(XEN) ACPI:                  wakeup_vec[cf6ab00c], vec_size[20]
(XEN) ACPI: Local APIC address 0xfee00000
(XEN) ACPI: LAPIC (acpi_id[0x01] lapic_id[0x00] enabled)
(XEN) Processor #0 6:15 APIC version 20
(XEN) ACPI: LAPIC (acpi_id[0x02] lapic_id[0x01] enabled)
(XEN) Processor #1 6:15 APIC version 20
(XEN) ACPI: LAPIC (acpi_id[0x03] lapic_id[0x82] disabled)
(XEN) ACPI: LAPIC (acpi_id[0x04] lapic_id[0x83] disabled)
(XEN) ACPI: LAPIC_NMI (acpi_id[0x01] dfl dfl lint[0x1])
(XEN) ACPI: LAPIC_NMI (acpi_id[0x02] dfl dfl lint[0x1])
(XEN) ACPI: IOAPIC (id[0x02] address[0xfec00000] gsi_base[0])
(XEN) IOAPIC[0]: apic_id 2, version 32, address 0xfec00000, GSI 0-23
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 0 global_irq 2 dfl dfl)
(XEN) ACPI: INT_SRC_OVR (bus 0 bus_irq 9 global_irq 9 high level)
(XEN) ACPI: IRQ0 used by override.
(XEN) ACPI: IRQ2 used by override.
(XEN) ACPI: IRQ9 used by override.
(XEN) Enabling APIC mode:  Flat.  Using 1 I/O APICs
(XEN) PCI: MCFG configuration 0: base f0000000 segment 0 buses 0 - 127
(XEN) PCI: Not using MMCONFIG.
(XEN) Table is not found!
(XEN) Using ACPI (MADT) for SMP configuration information
(XEN) IRQ limits: 24 GSI, 376 MSI/MSI-X
(XEN) Using scheduler: SMP Credit Scheduler (credit)
(XEN) Detected 2131.263 MHz processor.
(XEN) Initing memory sharing.
(XEN) mce_intel.c:1162: MCA Capability: BCAST 1 SER 0 CMCI 0 firstbank 1 extended MCE MSR 0
(XEN) Intel machine check reporting enabled
(XEN) I/O virtualisation disabled
(XEN) ENABLING IO-APIC IRQs
(XEN)  -> Using new ACK method
(XEN) ..TIMER: vector=0xF0 apic1=0 pin1=2 apic2=-1 pin2=-1
(XEN) Platform timer is 3.579MHz ACPI PM Timer
�(XEN) Allocated console ring of 16 KiB.
(XEN) VMX: Supported advanced features:
(XEN)  - APIC TPR shadow
(XEN)  - MSR direct-access bitmap
(XEN) HVM: ASIDs disabled.
(XEN) HVM: VMX enabled
(XEN) Brought up 2 CPUs
(XEN) CPUIDLE: disabled due to no HPET. Force enable with 'cpuidle'.
(XEN) ACPI sleep modes: S3
(XEN) mcheck_poll: Machine check polling timer started.
(XEN) *** LOADING DOMAIN 0 ***
(XEN)  Xen  kernel: 64-bit, lsb, compat32
(XEN)  Dom0 kernel: 64-bit, PAE, lsb, paddr 0x1000000 -> 0x2060000
(XEN) PHYSICAL MEMORY ARRANGEMENT:
(XEN)  Dom0 alloc.:   0000000218000000->000000021c000000 (1978543 pages to be allocated)
(XEN)  Init. ramdisk: 00000002258b5000->0000000227fff400
(XEN) VIRTUAL MEMORY ARRANGEMENT:
(XEN)  Loaded kernel: ffffffff81000000->ffffffff82060000
(XEN)  Init. ramdisk: ffffffff82060000->ffffffff847aa400
(XEN)  Phys-Mach map: ffffffff847ab000->ffffffff856f6fd0
(XEN)  Start info:    ffffffff856f7000->ffffffff856f74b4
(XEN)  Page tables:   ffffffff856f8000->ffffffff85727000
(XEN)  Boot stack:    ffffffff85727000->ffffffff85728000
(XEN)  TOTAL:         ffffffff80000000->ffffffff85800000
(XEN)  ENTRY ADDRESS: ffffffff81cfc200
(XEN) Dom0 has maximum 2 VCPUs
(XEN) Scrubbing Free RAM: .done.
(XEN) Xen trace buffers: disabled
(XEN) Std. Loglevel: All
(XEN) Guest Loglevel: All
(XEN) **********************************************
(XEN) ******* WARNING: CONSOLE OUTPUT IS SYNCHRONOUS
(XEN) ******* This option is intended to aid debugging of Xen by ensuring
(XEN) ******* that all output is synchronously delivered on the serial line.
(XEN) ******* However it can introduce SIGNIFICANT latencies and affect
(XEN) ******* timekeeping. It is NOT recommended for production use!
(XEN) **********************************************
(XEN) 3... 2... 1... 
(XEN) Xen is relinquishing VGA console.
(XEN) *** Serial input -> DOM0 (type 'CTRL-a' three times to switch input to Xen)
(XEN) Freed 216kB init memory.
mapping kernel into physical memory
Xen: setup ISA identity maps
about to get started...
[    0.000000] Initializing cgroup subsys cpuset
[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Linux version 3.2.0-36-generic (buildd@allspice) (gcc version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5) ) #57-Ubuntu SMP Tue Jan 8 21:44:52 UTC 2013 (Ubuntu 3.2.0-36.57-generic 3.2.35)
[    0.000000] Command line: placeholder root=UUID=4dcc2078-0a78-43eb-8ebc-83e6d17e3c16 ro xencons=hvc console=hvc0 console=tty0 earlyprintk=xen debug loglevel=8
[    0.000000] KERNEL supported cpus:
[    0.000000]   Intel GenuineIntel
[    0.000000]   AMD AuthenticAMD
[    0.000000]   Centaur CentaurHauls
[    0.000000] Freeing  8f-100 pfn range: 113 pages freed
[    0.000000] Freeing  ce674-ce6cc pfn range: 88 pages freed
[    0.000000] Freeing  cf5fb-cf608 pfn range: 13 pages freed
[    0.000000] Freeing  cf6a6-cf6f2 pfn range: 76 pages freed
[    0.000000] Freeing  cf6f3-cf6ff pfn range: 12 pages freed
[    0.000000] Freeing  cf700-100000 pfn range: 198912 pages freed
[    0.000000] Released 199214 pages of unused memory
[    0.000000] Set 215598 page(s) to 1-1 mapping
[    0.000000] BIOS-provided physical RAM map:
[    0.000000]  Xen: 0000000000000000 - 000000000008f000 (usable)
[    0.000000]  Xen: 000000000008f000 - 0000000000100000 (reserved)
[    0.000000]  Xen: 0000000000100000 - 00000000ce674000 (usable)
[    0.000000]  Xen: 00000000ce674000 - 00000000ce6cc000 (ACPI NVS)
[    0.000000]  Xen: 00000000ce6cc000 - 00000000cf5fb000 (usable)
[    0.000000]  Xen: 00000000cf5fb000 - 00000000cf608000 (reserved)
[    0.000000]  Xen: 00000000cf608000 - 00000000cf6a6000 (usable)
[    0.000000]  Xen: 00000000cf6a6000 - 00000000cf6ab000 (ACPI data)
[    0.000000]  Xen: 00000000cf6ab000 - 00000000cf6f2000 (ACPI NVS)
[    0.000000]  Xen: 00000000cf6f2000 - 00000000cf6f3000 (usable)
[    0.000000]  Xen: 00000000cf6f3000 - 00000000cf6ff000 (ACPI data)
[    0.000000]  Xen: 00000000cf6ff000 - 00000000cf700000 (usable)
[    0.000000]  Xen: 00000000cf700000 - 00000000d0000000 (reserved)
[    0.000000]  Xen: 00000000fec00000 - 00000000fec01000 (reserved)
[    0.000000]  Xen: 00000000fee00000 - 00000000fee01000 (reserved)
[    0.000000]  Xen: 00000000fff00000 - 0000000100000000 (reserved)
[    0.000000]  Xen: 0000000100000000 - 0000000228000000 (usable)
[    0.000000]  Xen: 0000000228000000 - 000000022c000000 (unusable)
[    0.000000] bootconsole [xenboot0] enabled
[    0.000000] NX (Execute Disable) protection: active
[    0.000000] DMI 2.4 present.
[    0.000000] DMI:                  /DG965SS, BIOS MQ96510J.86A.1669.2007.0406.0107 04/06/2007
[    0.000000] e820 update range: 0000000000000000 - 0000000000010000 (usable) ==> (reserved)
[    0.000000] e820 remove range: 00000000000a0000 - 0000000000100000 (usable)
[    0.000000] No AGP bridge found
[    0.000000] last_pfn = 0x228000 max_arch_pfn = 0x400000000
[    0.000000] last_pfn = 0xcf700 max_arch_pfn = 0x400000000
[    0.000000] initial memory mapped : 0 - 047ab000
[    0.000000] Base memory trampoline at [ffff88000008a000] 8a000 size 20480
[    0.000000] init_memory_mapping: 0000000000000000-00000000cf700000
[    0.000000]  0000000000 - 00cf700000 page 4k
[    0.000000] kernel direct mapping tables up to cf700000 @ 97f000-1000000
[    0.000000] xen: setting RW the range fce000 - 1000000
[    0.000000] init_memory_mapping: 0000000100000000-0000000228000000
[    0.000000]  0100000000 - 0228000000 page 4k
[    0.000000] kernel direct mapping tables up to 228000000 @ cecb5000-cf5fb000
[    0.000000] xen: setting RW the range cf5fa000 - cf5fb000
[    0.000000] init_memory_mapping: 0000000228000000-000000022c000000
[    0.000000]  0228000000 - 022c000000 page 4k
[    0.000000] kernel direct mapping tables up to 22c000000 @ 1e97d8000-1e97fa000
(XEN) mm.c:825:d0 Non-privileged (0) attempt to map I/O space 00228000
(XEN) mm.c:1222:d0 Failure in alloc_l1_table: entry 0
(XEN) mm.c:2177:d0 Error while validating mfn 81de (pfn 1e97d8) for type 1000000000000000: caf=8000000000000003 taf=1000000000000001
(XEN) mm.c:2985:d0 Error while pinning mfn 81de
(XEN) traps.c:451:d0 Unhandled invalid opcode fault/trap [#6] on VCPU 0 [ec=0000]
(XEN) domain_crash_sync called from entry.S
(XEN) Domain 0 (vcpu#0) crashed on cpu#0:
(XEN) ----[ Xen-4.1.2  x86_64  debug=n  Tainted:    C ]----
(XEN) CPU:    0
(XEN) RIP:    e033:[<ffffffff816415e1>]
(XEN) RFLAGS: 0000000000000282   EM: 1   CONTEXT: pv guest
(XEN) rax: 00000000ffffffea   rbx: 00000000001e97d8   rcx: 0000000000000007
(XEN) rdx: 0000000000000000   rsi: 0000000000000001   rdi: ffffffff81c01c30
(XEN) rbp: ffffffff81c01c48   rsp: ffffffff81c01be8   r8:  00003ffffffff000
(XEN) r9:  ffff880000000000   r10: 0000000000007ff0   r11: 0000000000000000
(XEN) r12: 0000000228000000   r13: 0000000000000000   r14: 0000000000000000
(XEN) r15: 0000000000000140   cr0: 000000008005003b   cr4: 00000000000026f0
(XEN) cr3: 0000000219c05000   cr2: 0000000000000000
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e02b   cs: e033
(XEN) Guest stack trace from rsp=ffffffff81c01be8:
(XEN)    0000000000000007 0000000000000000 ffffffff816415e1 000000010000e030
(XEN)    0000000000010082 ffffffff81c01c28 000000000000e02b ffffffff816415dd
(XEN)    ffffffff81c01c58 0000000100000000 00000000000081de 00000000001e97d8
(XEN)    ffffffff81c01c68 ffffffff81d008bf 00000001e97d8000 ffffffffff4b8a00
(XEN)    ffffffff81c01d08 ffffffff8163f25d 00000000cf4b9000 ffffffffff4f8000
(XEN)    ffff8801e97d8000 80000000000001e3 0000000228200000 ffffffffff4b8000
(XEN)    8000000000000163 000000022c000000 0000000000000140 0000000000000000
(XEN)    0000000000000000 00000001e97d8000 ffffffff81c01ce8 ffffffffff4b8000
(XEN)    0000000228000000 0000000000000000 0000000000000000 0000000000000008
(XEN)    ffffffff81c01d98 ffffffff8163f3d0 ffffffff81c01d68 8000000000000163
(XEN)    ffff88022c000000 0000000000000000 000000022c000000 ffffffffff478000
(XEN)    0000000000000000 000000022c000000 0000000000000000 000000022c000000
(XEN)    ffffffff81c01d78 ffff88022c000000 ffff88022c000000 ffffffffff478000
(XEN)    0000000000000000 000000022c000000 ffffffff81c01e08 ffffffff8163f626
(XEN)    0000000000022000 ffff880228000000 ffff880228000000 0000000000000000
(XEN)    0000000000001000 00000001e97fa000 ffffffff81c01e08 0000000000000000
(XEN)    ffffffff81c01e48 ffffffff81c01e48 ffffffff81c01e48 ffffffff81a2bf39
(XEN)    ffffffff81c01ed8 ffffffff816240ca 000000205d303030 000000022c000000
(XEN)    0000000000000001 0000000228000000 000000022c000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 0000000000000000 0000000000000000
(XEN) Domain 0 crashed: 'noreboot' set - not rebooting.