>timers through the direct Xen shared_info page
HVM has support PV timers (and interrupt controllers, and spinlocks) for quite some time now
>generally as "native" to x86 as Xen permits.
I really don't know if I agree with this. With hardware extensions basic CPU performance is going to be significantly better on HVM instances (No longer having to bounce to the hypervisor every time you make a system call since you once again have three CPU protection rings with ring -1), and that's before getting into SR-IOV, etc.
Unikernels are sweet and all, but without PVH, PV will outperform modern "PVHVM" implementations, and with PVH you're still running in a partial HVM shell.
The useful aspect about the MirageOS unikernels is that they use the pure Xen PV interface, which has almost no dependency on qemu. No floppy or block/net emulation, timers through the direct Xen shared_info page, and generally as "native" to x86 as Xen permits.