HVM, at least in the past, had a bunch more code that the guest DomU interacts with vs. fully pv guests. This has security implications.
Now, my knowledge of HVM is a few years... or more like half a decade out of date, for example, I don't even know how to force a HVM guest to only use PV drivers (which would solve 90% of the problem.) and i know that more and more of this has moved into hardware, so it's possible that what was true five years ago is not true now, but... yeah, I don't let untrusted users on HVM guests for the same reason I don't let untrusted users use pygrub or load untrusted kernels directly.