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.

http://www.howtoforge.com/centos_5.0_xen