I agree with VMware's position that paravirtualization adds management overhead to a Virtualized Datacenter. I would go one step further, and question the need for installing paravirtualized drivers of a specific variety (vmware or xen) in a guest OS. The biggest problem with this is that you cannot move a VM from a VMware VMM to a Xen VMM. This is harmful to customers. It is the responsibility of VMM vendors to make completely unmodified Guest OSes run well.

The VMware VMI interface was designed to make a cross platform standards based interface from hypervisors to guest Operating Systems. While this looks good on paper, I'm skeptical of its practicality. The x86 CPU's Machine Language and the PC Architecture form the best interface from Guest Operating Systems to hypervisors. Intel and AMD have a fanatical approach to backward compatiblity. That is why you can even run MS-DOS, a twenty year old OS on any modern PC. This level of attention to backward compatibility is hard to find in software companies. Without this level of backward compatibility, the greatest benefit of Virtualization, i.e. independence from the underlying hardware, will be reduced.