VM interfaces are awkward due to emulating hardware so that they can run existing operating systems. A VT-backed unikernel interface does not need to inherit this awkwardness.

As for memory management, in the Solo5/ukvm case the amount of memory you grant to the guest (monitor process) is essentially the maximum you expect the guest to need, ever. So it's similar in terms of overhead to many usermode allocators which just mmap() a huge block of memory at start and then hand out to the application from that.