Before PVH virtualization mode introduced (by Mukesh Rathor @Oracle in 2012), Xen (on x86) supports different virtualization modes, like PV, HVM, HVM with PV drivers, PVHVM depending the guest domain/OS type and hardware machine capability. This was pretty complicated in XEN design. I think we wouldn't do it like that if XEN on x86 project were launched in recent year (instead of 10 years ago). This is why XEN on ARM can do it better in this area.
Here are some very great posts that explain why PVH mode is much better than any one of previous virtualization modes based upon the latest x86 processors and platforms.
What Color Is Your Xen?
The Paravirtualization Spectrum, part 1: The Ends of the Spectrum
The Paravirtualization Spectrum, Part 2: From poles to a spectrum
At a glance, this picture below (from What Color Is Your Xen?) has a straightforward illustration of the differences among all those virtualization working modes.