[CentOS] Sharing RAM between VM

Hi, all!
I have a server with 4 virtual machines ( using xen). Is there a way to
make them share RAM? I found out something about balloons, but it didn't
distribute RAM dynamically. So I decided that xen can't achieve that. 
I tried luci and ricci and now all my VMs are in a cluster. But I don't
think this actually helped.
Can you suggest a software and/or manual or a question which I should
ask google for completing this task? 

All doms/nodes are with Centos 5.3(final).

Thanks!