1. A method of backing up a virtual machine, the method comprising: intercepting, by a first driver under the control of a first hypervisor which executes on a first client computing device, a first write operation generated by a virtual machine (VM) to store data in a first sector of a virtual hard disk, wherein the VM is executed by the first hypervisor, wherein the VM comprises a virtual hard disk file and a change block bitmap file, and wherein the virtual hard disk file stores data associated with the virtual hard disk; with the first client computing device, accessing the change block bitmap file and modifying a first entry in the change block bitmap file to indicate that data in the first sector has changed; performing a first migration of the VM to a second hypervisor which executes on a second client computing device, such that the VM is executed by the second hypervisor following said performing the first migration; performing a second migration of the VM either back to the first hypervisor or to a third hypervisor executing on a third client computing device; subsequent to said performing the second migration, receiving an instruction to begin an incremental backup of the VM; and in response to the instruction to begin the incremental backup of the VM: determining that changes made to the virtual machine hard disk file are not reflected in the change block bit map file, wherein the changes not reflected in the change block bit map file were made to the virtual machine hard disk file when the VM was executed by the second hypervisor, invalidating the change block bit map file, and performing a full backup of the VM.