Energy saving virtualization technique embodiments are presented that save energy by virtualizing a network-connected client computer's computing session (or a part thereof) as a virtual machine (VM) and then migrating it between the client computer and a VM server, depending on whether the computing session is active or idle. The client computer VM is migrated to the VM server when the client computer is deemed to be idle, and the VM is migrated back when the client computer is deemed to have re-entered an active condition. When the VM has been migrated away from the client computer and is residing on the VM server, the client computer is put into a sleep mode, thereby saving energy. However, the user's computing session remains alive on the VM server so that ongoing network connections and other activity are not disturbed.