xhyve needs to interact with all kinds of low-level things so there has to be kernel code involved. xhyve does not install kernel extensions of its own like VirtualBox or VMWare Fusion. xhyve uses the kernel extensions provided by Apple (com.apple.driver.AppleHV and possibly others).

Ah, that makes sense. It doesn't install anything extensions of it's own, only using the kernel bits provided by Hypervisor.framework.