Is it essentially implementing a paravirtualized OS that runs the JS runtime? Presumably you have to allocate the memory for the VM up front, or how does that work?

on linux its kvm, on darwin its hypervisor.framework, the memory is not upfront, by default 256mb is lazy allocated, and ~35mb are used by both ant + the vm. the kernel is https://nanos.org with patches to get ant+networking running smoothly