You should be able to get an activation key from MSDN for the Enterprise edition that works fine to activate your installation over the Internet without changing editions. I used a partner activation key with Win7 Enterprise and it worked fine. It should be the same for MSDN. In fact, I can generate a key in the MSDN subscriber downloads website also.
The main difference between all those solutions is the same reason as why there are so many car companies - it is a big market and everyone thinks they can do it better.
As for your comparison points, everyone will give different answers. Typically, if you just want to mess around and know what you are doing, I would recommend Virtual Box. If you want to do more advanced things, I would recommend VMWare Workstation, if you do not know what you are doing, but just want to virtualise, I would recommend Microsoft Virtual PC.
Qemu is not virtualisation, it is emulation. this means it is slower, but it has its moments and its niche that other products do not fit in to.
As for your direct points:
Virtualization
The process of running a machine virtually, this is the general name of the topic.
Virtual Machine
A name for a single machine that runs virtually on a host.
Hypervisor
A hypervisor is a virtualisation technology which allows for the greatest performance by attempting to run as thin as possible on top of the bare metal hardware.
Paravirtualization
This term is very awkward and is subjective to different people. Typically, most modern virtualisation technology, even those that use a Hypervisor also use paravirtualisation for a few hardware components (e.g. networking controller.). My definition (which I am happy to be corrected on) is presenting virtual hardware which is different to the physical hardware it is actually connected to.
Hardware Virtualization
(typically) Support for virtualisation is built in to the processor directly which increases the performance. For example, in the early days of virtualisation, it was performed via emulation which has extra overhead. Hardware virtualisation is basically a bunch of "shortcuts" inside the processor that allow virtual machines to run a lot faster - in some cases, almost at the same speed as a native machine.
I hope this helps!
Best Answer
http://en.wikipedia.org/wiki/Windows_7_editions
Looks like it is just a difference in licensing between Enterprise and Ultimate.
Interesting to note that Vista Ultimate and Enterprise did have some actual differences in functionality (namely, no Windows Media Center or Ultimate Extras in the Enterprise edition): http://en.wikipedia.org/wiki/Windows_Vista_editions
UPDATE: And here is a thread with several MS MVPs confirming that the only difference between Windows 7 Ultimate and Enterprise is the licensing model.