From what I see, CentOS 5.3 comes with Xen 3.0.3 and Debian Lenny (assuming your using Lenny) comes with Xen 3.2.1 (I do see something about OpenVZ...though never heard of it). Possibly support for '08 was added later between 3.0 and 3.2.
I'd actually suggest using VMWare Server 2.x if you want free VM software.
edit: might of had better luck in the Linux section...