Doing xen. This is a rough summary of how I built the xen image from FC4. Also the code is going to be here so you can attempt to do the same.
(Work in progress!) First, you want to get mercurial on Linux so you can get xen. Do that and pull down xen via the commands at the xen website -- not xensource, the website at cambridge. Or, get lazy and pull down the tree I am putting here. Either will work. This tree has a prebuilt kernel etc. and to install, you cd to the top-level directory and do this KERNEL=linux-2.6.16.29-xen make install (This should work, but not tested yet. Corrections welcome)
now, the init file in the initrd has some issues. You have to fix it a little bit. Mainly, you don't want it
starting udev, since the config files are wrong. So you do this:
cd /tmp
mkdir initrd
cd inird
zcat /boot/initrd-2.6.16.29-xen.img | cpio -i
Now, edit the initrd and comment out the lines that involve starting udev.
Then,
find . -print | cpio -o > ../newinitrd
cd ..
gzip -9 newinitrd > /boot/initrd-2.6.16.29-xen.img
Now you have a working initrd.
You need to set up a grub entry, this assumes your partition is on /dev/hda4
title tiny horrible xen
root (hd0,3)
kernel /boot/xen-3.0.3-0.gz console=vga
module /boot/vmlinuz-2.6.16.29-xen root=/dev/hda4 ro console=tty0 init=/bin/bash
module /boot/initrd-2.6.16.29-xen.img
now reboot
once up in a bash prompt, follow the instructions in the README