Okay, here goes my tests:

2.6.35-28-server #50-Ubuntu SMP Fri Mar 18 18:59:25 UTC 2011 x86_64 GNU/Linux
[2819125.429932] ------------[ cut here ]------------
[2819125.429943] kernel BUG at /build/buildd/linux-2.6.35/arch/x86/xen/spinlock.c:344!
[2819125.429950] invalid opcode: 0000 [#1] SMP 
[2819125.429956] last sysfs file: /sys/kernel/uevent_seqnum
[2819125.429961] CPU 3 
[2819125.429964] Modules linked in: xenfs lp parport xen_netfront xen_blkfront
[2819125.429977] 
[2819125.429981] Pid: 12, comm: migration/3 Not tainted 2.6.35-28-server #50-Ubuntu /
[2819125.429987] RIP: e030:[<ffffffff81007d04>]  [<ffffffff81007d04>] dummy_handler+0x4/0x10
[2819125.430000] RSP: e02b:ffff880003f60ea8  EFLAGS: 00010046
[2819125.430005] RAX: ffffffffff57b000 RBX: ffff88001fc1a8a0 RCX: 0000000000000000
[2819125.430011] RDX: 0000000000400200 RSI: 0000000000000000 RDI: 0000000000000013
[2819125.430016] RBP: ffff880003f60ea8 R08: 0000000000000600 R09: 0000000000000000
[2819125.430022] R10: ffff880003f68028 R11: 0000000000012ed0 R12: 0000000000000000
[2819125.430028] R13: 0000000000000000 R14: 0000000000000013 R15: 0000000000000100
[2819125.430038] FS:  00007f2137153700(0000) GS:ffff880003f5d000(0000) knlGS:0000000000000000
[2819125.430045] CS:  e033 DS: 0000 ES: 0000 CR0: 000000008005003b
[2819125.430050] CR2: 0000000000000000 CR3: 0000000003acf000 CR4: 0000000000002660
[2819125.430057] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[2819125.430063] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[2819125.430069] Process migration/3 (pid: 12, threadinfo ffff88001fdd2000, task ffff88001fd9dbc0)
[2819125.430076] Stack:
[2819125.430079]  ffff880003f60ef8 ffffffff810ca310 0000000000000000 000000000072f59a
[2819125.430088] <0> ffff880003f60ef8 ffff88001fc05900 0000000000000013 0000000000000600
[2819125.430099] <0> 0000000000000001 0000000000000100 ffff880003f60f18 ffffffff810cca52
[2819125.430111] Call Trace:
[2819125.430115]  <IRQ> 
[2819125.430123]  [<ffffffff810ca310>] handle_IRQ_event+0x50/0x160
[2819125.430130]  [<ffffffff810cca52>] handle_percpu_irq+0x42/0x80
[2819125.430139]  [<ffffffff81348706>] xen_evtchn_do_upcall+0x1d6/0x200
[2819125.430147]  [<ffffffff810b2400>] ? stop_machine_cpu_stop+0x0/0xe0
[2819125.430154]  [<ffffffff8100b02e>] xen_do_hypervisor_callback+0x1e/0x30
[2819125.430159]  <EOI> 
[2819125.430164]  [<ffffffff810b2400>] ? stop_machine_cpu_stop+0x0/0xe0
[2819125.430172]  [<ffffffff8100122a>] ? hypercall_page+0x22a/0x1010
[2819125.430179]  [<ffffffff8100122a>] ? hypercall_page+0x22a/0x1010
[2819125.430186]  [<ffffffff81006b9d>] ? xen_force_evtchn_callback+0xd/0x10
[2819125.430192]  [<ffffffff81007332>] ? check_events+0x12/0x20
[2819125.430199]  [<ffffffff810072d9>] ? xen_irq_enable_direct_end+0x0/0x7
[2819125.430206]  [<ffffffff810b2301>] ? cpu_stopper_thread+0xd1/0x1d0
[2819125.430214]  [<ffffffff8159f6f1>] ? schedule+0x3e1/0x830
[2819125.430221]  [<ffffffff815a1aee>] ? _raw_spin_unlock_irqrestore+0x1e/0x30
[2819125.430227]  [<ffffffff810b2230>] ? cpu_stopper_thread+0x0/0x1d0
[2819125.430235]  [<ffffffff8107f616>] ? kthread+0x96/0xa0
[2819125.430240]  [<ffffffff8100aee4>] ? kernel_thread_helper+0x4/0x10
[2819125.430247]  [<ffffffff8100a313>] ? int_ret_from_sys_call+0x7/0x1b
[2819125.430254]  [<ffffffff815a205d>] ? retint_restore_args+0x5/0x6
[2819125.430260]  [<ffffffff8100aee0>] ? kernel_thread_helper+0x0/0x10
[2819125.430265] Code: 89 e5 c9 0f 95 c0 c3 55 b8 01 00 00 00 86 07 84 c0 48 89 e5 0f 94 c0 c9 0f b6 c0 c3 eb 09 90 90 90 90 90 90 90 90 90 55 48 89 e5 <0f> 0b eb fe eb 06 90 90 90 90 90 90 55 ba ff ff ff ff 48 89 e5 
[2819125.430341] RIP  [<ffffffff81007d04>] dummy_handler+0x4/0x10
[2819125.430348]  RSP <ffff880003f60ea8>
[2819125.430354] ---[ end trace d7c57d1d217dc8cc ]---
[2819125.430358] Kernel panic - not syncing: Fatal exception in interrupt
[2819125.430364] Pid: 12, comm: migration/3 Tainted: G      D     2.6.35-28-server #50-Ubuntu
[2819125.430370] Call Trace:
[2819125.430374]  <IRQ>  [<ffffffff8159ed8a>] panic+0x90/0x113
[2819125.430383]  [<ffffffff815a1aee>] ? _raw_spin_unlock_irqrestore+0x1e/0x30
[2819125.430390]  [<ffffffff815a301a>] oops_end+0xea/0xf0
[2819125.430396]  [<ffffffff8100de0b>] die+0x5b/0x90
[2819125.430402]  [<ffffffff815a2884>] do_trap+0xc4/0x170
[2819125.430408]  [<ffffffff8100ba35>] do_invalid_op+0x95/0xb0
[2819125.430414]  [<ffffffff81007d04>] ? dummy_handler+0x4/0x10
[2819125.430420]  [<ffffffff81006b9d>] ? xen_force_evtchn_callback+0xd/0x10
[2819125.430426]  [<ffffffff81007332>] ? check_events+0x12/0x20
[2819125.430432]  [<ffffffff8100ad5b>] invalid_op+0x1b/0x20
[2819125.430439]  [<ffffffff81007d04>] ? dummy_handler+0x4/0x10
[2819125.430445]  [<ffffffff810ca517>] ? irq_to_desc+0x17/0x20
[2819125.430450]  [<ffffffff810ca310>] handle_IRQ_event+0x50/0x160
[2819125.430457]  [<ffffffff810cca52>] handle_percpu_irq+0x42/0x80
[2819125.430463]  [<ffffffff81348706>] xen_evtchn_do_upcall+0x1d6/0x200
[2819125.529453]  [<ffffffff810b2400>] ? stop_machine_cpu_stop+0x0/0xe0
[2819125.529464]  [<ffffffff8100b02e>] xen_do_hypervisor_callback+0x1e/0x30
[2819125.529469]  <EOI>  [<ffffffff810b2400>] ? stop_machine_cpu_stop+0x0/0xe0
[2819125.529481]  [<ffffffff8100122a>] ? hypercall_page+0x22a/0x1010
[2819125.529488]  [<ffffffff8100122a>] ? hypercall_page+0x22a/0x1010
[2819125.529495]  [<ffffffff81006b9d>] ? xen_force_evtchn_callback+0xd/0x10
[2819125.529501]  [<ffffffff81007332>] ? check_events+0x12/0x20
[2819125.529508]  [<ffffffff810072d9>] ? xen_irq_enable_direct_end+0x0/0x7
[2819125.529514]  [<ffffffff810b2301>] ? cpu_stopper_thread+0xd1/0x1d0
[2819125.529523]  [<ffffffff8159f6f1>] ? schedule+0x3e1/0x830
[2819125.529530]  [<ffffffff815a1aee>] ? _raw_spin_unlock_irqrestore+0x1e/0x30
[2819125.529536]  [<ffffffff810b2230>] ? cpu_stopper_thread+0x0/0x1d0
[2819125.529543]  [<ffffffff8107f616>] ? kthread+0x96/0xa0
[2819125.529549]  [<ffffffff8100aee4>] ? kernel_thread_helper+0x4/0x10
[2819125.529556]  [<ffffffff8100a313>] ? int_ret_from_sys_call+0x7/0x1b
[2819125.529563]  [<ffffffff815a205d>] ? retint_restore_args+0x5/0x6
[2819125.529569]  [<ffffffff8100aee0>] ? kernel_thread_helper+0x0/0x10

2.6.35-30-server #54-Ubuntu SMP Tue Jun 7 20:13:05 UTC 2011 x86_64 GNU/Linux
..... Working fine !!

Tks !

At least in my case, lucid still have some glitches while xenmotion, but its much better with the latest kernel release.

Maverick seems to be working fine using the 35-30 proposed kernel. All 8 xenmotions to different hosts worked, no fail.