1. First we need to add the YUM repository holding the updated Xen. You will need to be logged in as root to carry out these instructions
wget http://www.gitco.de/linux/i386/centos/5/CentOS-GITCO.repo -O /etc/yum.repos.d/CentOS-GITCO.repo
2. Uninstall and reinstall the Virtualization group
yum groupremove Virtualization
yum update
yum clean all
yum groupinstall -y Virtualization
Warning! If you get an error message from grubby this is bad!
Installing: kernel-xen ####################### [ 9/13]
grubby fatal error: unable to find a suitable template
This means that your grub.conf file couldn't be written to for whaterver reason. I'm not sure yet why this is happening but it basically means the grub.conf will be pointing to your old xen-kernel instead of your new one so you won't be able to successfully reboot. If you get this message you need to edit your /boot/grub/grub.conf file and make the kernel lines match the kernel you installed. To get your installed xen-kernel version check it with rpm.
[ root@vs / ] rpm -q kernel-xen
kernel-xen-2.6.18-92.1.6.el5
Now edit your /boot/grub/grub.conf to match
# grub.conf generated by anaconda
#
# Note that you do not have to rerun grub after making changes to this file
# NOTICE: You have a /boot partition. This means that
# all kernel and initrd paths are relative to /boot/, eg.
# root (hd0,0)
# kernel /vmlinuz-version ro root=/dev/vgsys/lvroot
# initrd /initrd-version.img
#boot=/dev/sda
default=0
timeout=5
splashimage=(hd0,0)/grub/splash.xpm.gz
hiddenmenu
title CentOS (2.6.18-92.1.6.el5xen)
root (hd0,0)
kernel /xen.gz-3.3.0
module /vmlinuz-2.6.18-92.1.6.el5xen ro root=/dev/vgsys/lvroot rhgb quiet
module /initrd-2.6.18-92.1.6.el5xen.img
4. Reboot - no really I mean it.
5. Try it out by using the xm dmesg command
[ root@vs ~ ] xm dmesg
__ __ _____ _ _ ___
\ \/ /___ _ __ |___ /| || | / _ \
\ // _ \ '_ \ |_ \| || |_| | | |
/ \ __/ | | | ___) |__ _| |_| |
/_/\_\___|_| |_| |____(_) |_|(_)___/
(XEN) Xen version 3.4.0 (
That's about all. If you have any questions drop a comment here.