There is not yet a xen-amd64 domU kernel for Lenny. There might not be one on release day. Discussion here: http://groups.google.com/group/linux.debian.devel/browse_thread/thread/8940430a093bbda5/546883797d0cbdf9

Fortunately, someone builds a 2.6.26-xen-amd64 kernel.
Add this to your sources.list:
deb http://kernel-archive.buildserver.net/debian-kernel/waldi/xen-extra/ all main

Install these packages:
linux-image-2.6.26-1-xen-amd64
linux-modules-2.6.26-1-xen-amd64

Fix the “new Xen console” problem (xen now uses hvc0 for its console):
If you use pygrub, in grub.conf on the domU add these kernel parameters : console=hvc0 xencons=tty
title Debian 2.6.26
kernel /boot/vmlinuz-2.6.26-1-xen-amd64 ro root=/dev/sda1 console=hvc0 xencons=tty
initrd /boot/initrd.img-2.6.26-1-xen-amd64

If you don’t use pygrub, specify extra = “console=hvc0 xencons=tty” in your virtual machine xen config file.

Use at your own risk 😉

By the way, this fixed a nasty bug for me: php5-cli used to segfault a lot when using linux-image-2.6.18-6-xen-amd64 under Lenny. It’s stable now.

Edit: linux-image-2.6.26-1-xen-amd64 is now available in unstable 🙂