1. In a networked system of computers, including a source computer and a destination computer and a source virtual machine (VM) installed on the source computer, a virtualization method for migrating the source VM to a destination VM, the method comprising: pre-copying source VM memory to the destination VM; suspending execution of the source VM; transferring non-memory source VM state information to the destination VM, wherein the pre-copying of source VM memory is done before transferring the non-memory source VM state information; running the destination VM from the transferred non-memory source VM state; and after transferring the non-memory source VM state information, retransferring to the destination VM one or more units of the source VM memory that the source VM has modified during the interval between when the memory units were pre-copied to the destination VM and when the non-memory source VM state information was transferred.