It may be interesting to consider the xl save and xl restore commands: saving a domU frees the resources that were previously used by this domU, including RAM.

Note that even though a paused domU does not use any processor power, its allocated memory is still in use. For instance, a domU can be temporarily paused then resumed, with the xl pause and xl unpause commands. However, its virtual machine status allows some extra features. Once the domU is up, it can be used just like any other server (since it is a GNU/Linux system after all).