As Belgarath says, it takes xen-4.15+

tlviewer@xen2204:~$ sudo xl list
Name                                        ID   Mem VCPUs	State	Time(s)
Domain-0                                     0   600     2     r-----      45.8
ub2004                                       1   804     2     -b----      10.7
ub2204                                       2   360     2     -b----       9.2
fc34min                                      3   604     2     -b----      22.8
al8pv                                        4   854     2     -b----      13.4
fc35min                                      5   784     2     -b----      28.3

sudo systemd-analyze time
Startup finished in 8.070s (kernel) + 32.310s (userspace) = 40.380s 
multi-user.target reached after 32.271s in userspace

tlviewer@xen2204:~$ sudo xl info
host                   : xen2204
release                : 5.13.0-19-generic
version                : #19-Ubuntu SMP Thu Oct 7 21:58:00 UTC 2021
machine                : x86_64
nr_cpus                : 2
max_cpu_id             : 3
nr_nodes               : 1
cores_per_socket       : 2
threads_per_core       : 1
cpu_mhz                : 3062.672
hw_caps                : bfebfbff:0008e3bd:20100800:00000001:00000000:00000000:00000000:00000000
virt_caps              : pv hvm shadow
total_memory           : 4094
free_memory            : 36
sharing_freed_memory   : 0
sharing_used_memory    : 0
outstanding_claims     : 0
free_cpus              : 0
xen_major              : 4
xen_minor              : 15
xen_extra              : .1
xen_version            : 4.15.1
xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 hvm-3.0-x86_32p hvm-3.0-x86_64 
xen_scheduler          : credit2
xen_pagesize           : 4096
platform_params        : virt_start=0xffff800000000000
xen_changeset          : a763f8f158e81158b-x86/spec-ctrl: Fix default calculation of opt
xen_commandline        : dom0_mem=600M,max:648M guest_loglvl=all loglvl=all sched=credit2 console=vga vga=gfx-1280x1024x16 conring_size=48k xsave=0 xpti=false cpufreq=xen:performance
cc_compiler            : gcc (Ubuntu 11.2.0-13ubuntu1) 11.2.0
cc_compile_by          : pbuser
cc_compile_domain      : prymar56.org
cc_compile_date        : Sat Jan  8 13:42:29 PST 2022
build_id               : 3a9d3ef047996d4f6be41d9dadda2363d6538db7
xend_config_format     : 4

tlviewer@xen2204:~$ sudo dpkg -l | grep -P 'xen|qemuu'
ii  libxen-4.15:amd64                     4.15.1-1+ub22u04.2                amd64        Public libs for Xen
ii  libxenstore3.0:amd64                  4.15.1-1+ub22u04.2                amd64        Xenstore communications library for Xen
ii  libxentoolcore1:amd64                 4.15.1-1+ub22u04.2                amd64        helper for qemu & libxenstore
ii  qemuu                                 6.1.0+1-1+ub22u04.2               amd64        qemu-system-i386 (6.1.0/xen-4.15) with 9pfs support
ii  xen-hypervisor-4.15-amd64             4.15.1-1+ub22u04.2                amd64        Xen Hypervisor on AMD64
ii  xen-system-amd64                      4.15.1-1+ub22u04.2                amd64        Xen System on AMD64 (meta-package)
ii  xen-utils-4.15                        4.15.1-1+ub22u04.2                amd64        XEN administrative tools
ii  xen-utils-common                      4.15.1-1+ub22u04.2                all          Xen administrative tools - common files
ii  xenstore-utils                        4.15.1-1+ub22u04.2                amd64        Xenstore command line utilities for Xen

this is an official build using a delta similar to pre-Sep-2019 days applied to the full upstream tarball.