I have been looking to various GNU/Linux distributions to determine their level of support for Xen.  Here’s what I found:

• Debian Etch – Xen Hypervisor 3.2.1 (via backports), Linux kernel 2.6.26 (via backports)
• Debian Lenny – Xen Hypervisor 3.2.1, Linux dom0 kernel 2.6.26
• Debian Squeeze (testing) – Xen Hypervisor 3.4.2, Linux dom0 kernel ???
• Ubuntu 8.04 (Hardy) – Xen Hypervisor 3.3.0 (via backports), Linux dom0 kernel 2.6.24
• Ubuntu 8.10-10.04 (Intrepid-Lucid) – Xen Hypervisor 3.3.0, no Linux dom0 kernel
• OpenSuse 11.2 – Xen Hypervisor 3.4.1, Linux dom0 kernel 2.6.31

As much as I like Debian and Debian based distros I think OpenSuse is probably the best platform for hosting Xen at the moment.