This documentation describes a setup we have been working with in the last couple of years for hosting virtual machines on our Hetzner server. It is reflecting a setup on a prestine Debian 6 host. 
Once you have installed a fresh image of "Debian Squeeze 6.0 64Bit minimal" you need to make sure that your system is booting with a XEN enabled kernel. Therefore you need to change GRUB to reflect this:
  mv /etc/grub.d/10_linux /etc/grub.d/25_linux
  update-grub2
Update your system
  apt-get update
  apt-get dist-upgrade
Restart your server and then install XEN
 
  sudo shutdown -r now
  aptitude install xen-linux-system-amd64 (you might have to decline the first solution and accept the second solution to Upgrade to linux 3)
  apt-get install xen-qemu-dm-4.0

Enable NAT on the Dom 0 (your main host) by editing  /etc/xen/xend-config.sxp
(network-script network-nat)
(vif-script vif-nat)
 
Then get Cloudmin to work
wget http://cloudmin.virtualmin.com/gpl/scripts/cloudmin-gpl-debian-install.sh
sh cloudmin-gpl-debian-install.sh