XEN vs XENHVM?

[Mason Loring Bliss]

I was psyched to see that GENERIC kernels in 10 have HVMXEN support by
default, but then I was left a little confused.

What's the different between a kernel with options XEN and one with options
HVMXEN? I'd love to be able to run FreeBSD domU systems without having to do
a custom compile whenever there's an update. I've got a 9.1 system running
now, using a copy of the XEN config with a couple tweaks, and I see all the
PV drivers I expect. I'm wondering what's different with XENHVM... Also
useful would be knowing if there are remaining differences between i386 and
amd64 as a domU in FreeBSD 10.

Thanks in advance for clues.

-- 
The creatures outside looked from pig to man, and from man to pig, and from pig
to man again; but already it was impossible to say which was which. - G. Orwell