Howtoforge posted a guide about installing Xen 3.3 with Kernel 2.6.27 on Ubuntu 8.1

This tutorial shows how you can install Xen 3.3 on an Ubuntu 8.10 host (dom0). Xen 3.3 is available from the Ubuntu 8.10 repositories, but the Ubuntu 8.10 kernels (2.6.27-x) are domU kernels, i.e., they work for Xen guests (domU), but not for the host (dom0). Therefore we need to build our own dom0 kernel. This guide explains how to do this with a 2.6.27 kernel.

Installing Xen 3.3 With Kernel 2.6.27 On Ubuntu 8.10 (x86_64)