Then, you should be able to install the xen and kernel-xen0 packages. Once this is done, you should have an entry set up in your grub.conf to boot the xen0 kernel. Now, reboot into your new xen0 kernel [3]
title Xen
root (hd0,1)
kernel /boot/xen.gz dom0_mem=400000
module /boot/vmlinuz-2.6.10-xen0 ro root=/dev/hda2
module /boot/initrd-2.6.10-xen0.imgtitle Xen
root (hd0,1)
kernel /boot/vmlinuz-2.6.10-xen0 ro root=/dev/hda2
initrd /boot/initrd-2.6.10-xen0.img