Looks like Virtualization.framework is newer (Big Sur only) and uses Objective C classes rather than C functions.
I know from the xhyve README that it contains code responsible for booting a Linux kernel, which suggests Hypervisor.framework does not take care of that for you. The Virtualization.framework API on the other hand takes a linux kernel + ramdisk as its inputs.
So it sounds like the framework vftool is built on is more high-level than that of xhyve, and like vftool is Linux-only.
Yeah, Hypervisor.framework is an analogue of KVM (i.e. just manual VM contexts, the caller needs to handle exits to provide virtual hardware). Virtualization.framework is closer to something like QEMU in library form.