I have tried under Ubuntu 10.10 (Maverick Meerkat) to install Xen for virtualization. I was very surprised because APT-GET brought the message that the package "xen-tools" was not found.
I then looked in Synaptic, since the package was not available. In the package xen-tools
sources have been either forgotten or replaced by a new package. In this case, the developer would then forget the dependencies to strip properly.
The experiment package from the Debian package management failed as well, because the version was too outdated there. Next, I was maybe on the Xen tools web page, and there I found a Debian package in version 4.2, which lied to install it without problems on Ubuntu 10.10. Here is the link to the website: http://xen-tools.org/software/xen-tools/releases.html there, and you download a package (Debian package: xen-tools_4.2-1_all.deb - 254k, MD5 e315aad0528784350134b0b517b70649) and install it. Then you can install "ubuntu-xen-server" or "ubuntu-xen-desktop" problem via Synaptic or apt-get.
sudo aptitude search xen
?