There is a technique called "memory ballooning" http://www.vmwarearena.com/vmware-memory-management-part-3-memory-ballooning/ http://www.vmwarearena.com/vmware-

There is a technique called "memory ballooning" http://www.vmwarearena.com/vmware-memory-management-part-3-memory-ballooning/ http://www.vmwarearena.com/vmware-memory-management-part-3-m..., which is probably also used by HyperV to free memory again.