Xen has a bit of a weird background. Ever wondered why - for example - Fedora 11 cannot function as a Xen host, but only as a guest?
Xen development has always been out-of-kernel and requires massive patching your kernel if you use a a current one. CentOS (and RHEL, for that matter) are based on kernel 2.6.18, for which excellent Xen support exists, because Red Hat supports that specific kernel to be a Xen dom0.
Newer kernels than 2.6.18 can - and probably will - be problematic. Check this article on ubuntu.com, for example. Note that is not impossible to use Ubuntu as a Xen host, but I certainly wouldn't recommend it, and neither would, in the end, the author of the aforementioned article.
If you want to use a Debian based distribution for a Xen server, use Debian, not Ubuntu. That said, I would recommend a Red Hat based distribution (more specific CentOS) anytime, because of the fact that RHEL5 (and thus CentOS5) is an enterprise class distribution, which is marketed (amongst others) for it's Xen capabilities. RHEL5 (and thus CentOS5) will be around for a long time. Otoh, it is likely that Red Hat will drop Xen eventually. KVM seems to be the future.
Anyway, to make a long story short: go for CentOS5. Everything for a Xen server is in there, it is packaged very well and it just works.