Howtoforge posted a guide about installing Xen on CentOS 5.0

This tutorial provides step-by-step instructions on how to install Xen on a CentOS 5.0 system (i386). Xen lets you create guest operating systems (*nix operating systems like Linux and FreeBSD), so called virtual machines or domUs, under a host operating system (dom0). Using Xen you can separate your applications into different virtual machines that are totally independent from each other, but still use the same hardware.

Installing Xen On CentOS 5.0 (i386)