OpenVZ

http://openvz.org/Introduction_to_virtualization

OS-level virtualization systems have been designed to provide the required isolation and security to run multiple applications or copies of the same OS (but different distributions of the OS) on the same server.

So can one have Ubuntu 12.04 on Debian or on Ubuntu 14.04?

Ubuntu support

https://help.ubuntu.com/community/OpenVZ "Hardy release of Ubuntu, the OpenVZ packages are in the "universe" "

http://packages.ubuntu.com/search?keywords=OpenVZ+&searchon=names&suite=precise&section=all

no hit

http://packages.ubuntu.com/search?suite=precise&section=all&arch=any&keywords=OpenVZ&searchon=all

some hits

Debian support

http://www.heise.de/forum/c-t/Kommentare/Serverdaemmerung/Linux-Containers/posting-401631/show/

"OpenVZ geht nimmer mit Debian Wheezy; schon gehört? Debian bevormundet und beglückt seine User/Admins lieber mit Linux Containern ("LXC"),"