wget http://bits.xensource.com/oss-xen/release/3.0.4-1/bin.tgz/xen-3.0.4_1-install-x86_64.tgz
tar zxvf xen-3.0.4_1-install-x86_64.tgz
cd dist
sh ./install.sh
module_upgrade
depmod -ae -F System.map-2.6.16.33-xen  2.6.16.33-xen
mkinitrd -f --allow-missing /boot/initrd-2.6.16-33-xen.img 2.6.16.33-xen

Add entry to grub.conf:

title CentOS 4.4  with Xen (2.6.16.33-xen_3.0.4.1)
        root (hd0,0)
        kernel /xen-3.0.4-1.gz
        module /vmlinuz-2.6.16.33-xen ro root=/dev/vg_wast/lv_root
        module /initrd-2.6.16-33-xen.img