I have been trying to install a para-virtualized CentOS guest on my Debian machine using Xen 3.2.1. The xen-create-image script comes with support for rinse, which in turn can install CentOS and Fedora. Unfortunately the list of packages installed during setup is incomplete.You need to add the following package names to /etc/rinse/centos-5.packages :

libuser
nspr
nss
passwd
python-iniparse

This will allow you to set the root password and once you’re logged into the system yum will be operational.