No, but the memory accesses have hooks that say "This memory was written". Then, program execution is paused, and just the sections of memory that were written

No, but the memory accesses have hooks that say "This memory was written". Then, program execution is paused, and just the sections of memory that were written are copied again. This has memory performance characteristics - I ran a benchmark of memory read/write speed while this was happening once. It more than halved memory speed for the 30s or so it took from migration started to migration complete. The pause, too, was much longer.