running CentOS as a Guest OS on Ubuntu

http://wiki.centos.org/HowTos/Virtualization/Introduction

http://wiki.centos.org/HowTos/Xen/InstallingCentOSDomU

http://wiki.centos.org/HowTos/Xen/MoveNative2DomU

I had to add
ubuntu-virt-mgmt was needed.

Before you begin all this do check this page
https://help.ubuntu.com/community/KVM/Installation

egrep -c '(vmx|svm)' /proc/cpuinfo