Thanks for explaining; that helps a lot. I think of qemu as doing two separable things:
A) instruction-level emulation B) emulation of memory management, IO, etc. When you use qemu/nvmm or qemu/xen, you omit A, and just do B. Sounds like bhyve/vmm is the same. A key question is whether you can do IO via a hypercall that just asks for IO (xen xbd, virtio I think), or whether the virtual machine sees an AHCI or whatever controller. virtio is easier on the hypervisor side, but the guests need the driver. easy for Free Software, tricky for others, but AIUI the virtio interface is so standard that maybe it works with windows out of the box. If I got things right, I think it does make sense to see about runing bhyve on nvmm.
