Within the last few days balloon driver code has been added to the GPL repository of the KVM windows guest driver. Ballooning is just one technique of providing memory overcommit with server virtualization. Other techniques include page sharing, and swapping. The technique of ballooning requires cooperation between the guest and host where the guest is responsible for picking unused memory pages to evict. The ballooning feature is currently available via monitor commands on the host.

On the host side, the balloon driver has been available on linux since kernel version 2.6.27 and a recent enough version of kvm. On the guest side, only linux so far has a balloon driver capable of cooperating with kvm hosts for giving up unused memory. The balloon driver code recently added to the windows guest drivers is an an obvious indication that work is underway for windows guests but there is no indication of a timeline on when this will be available for testing.