A source virtual machine (VM) hosted on a source server is migrated to a destination VM on a destination server without first powering down the source VM. After optional pre-copying of the source VM's memory to the destination VM, the source VM is suspended and its non-memory state is transferred to the destination VM; the destination VM is then resumed from the transferred state. In one embodiment, the source VM memory is either paged into the destination VM on demand, or is transferred asynchronously by pre-copying and write-protecting the source VM memory, and then later transferring only the modified pages after the destination VM is resumed. In one embodiment, the source and destination servers share common storage, in which the source VM's virtual disk is stored; this avoids the need to transfer the virtual disk contents.