1. A method, comprising: running a Virtual Machine (VM) on a first compute node in a plurality of compute nodes that communicate with one another over a communication network; and migrating the VM from the first compute node to a second compute node in the plurality, by: generating, for memory pages accessed by the VM, page transfer state of one or more local memory pages that are accessed locally on the first compute node, and of one or more externalized memory pages whose access is not confined to the first node; based on the page transfer state, providing for the migrated VM access to the memory pages, including both the local and the externalized memory pages, on the second compute node; and transferring from the first compute node to the second compute node contents or unique identifiers of the local memory pages, further transferring from the first compute node to the second compute node unique identifiers of the externalized memory pages after stopping the VM on the first compute node, and transferring remaining memory pages after starting a migrated instance of the VM on the second compute node.