During the last days, the official Xen blog published two really amazing articles about Xen virtualization in detail.
Both of them take a look at paravirtualization, HVM and the difference between 32 and 64 Bit guests.
You can find part one of the article series here:
The Paravirtualization Spectrum, part 1: The Ends of the Spectrum
Part one covers the development of Xen in the early days and how HVM was born.
You can find part two of the article series here:
The Paravirtualization Spectrum, Part 2: From poles to a spectrum
Part two describes how other new virtualization modes, such as PVH, profit from mixing paravirtualization with full virtualization.
I highly recommend these articles for everyone being interested in Xen and virtualization in general.