Howtoforge posted a guide about installing Xen on Ubuntu 8.04 server

This tutorial provides step-by-step instructions on how to install Xen on an Ubuntu Hardy Heron (Ubuntu 8.04) server system (i386). You can find all the software used here in the Ubuntu repositories, so no external files (apart from a fixed Ubuntu Xen kernel to enable networking for the virtual machines) or compilation are needed.

Installing Xen On An Ubuntu 8.04 Server From The Ubuntu Repositories