When I bought what is now my primary laptop, I had intended to use the extra flexibility to learn the prevailing (industrial-grade) virtualization technology. While that project would have been edifying on its own, I also hoped to use the extra flexibility to some more consistent testing and development work.
This project spurned a xen laptop project, but the truth is that Xen is incredibly difficult to get working, and eventually the "new laptop" just became the "every day laptop," and I let go of the laptop Xen project. In fact, until very recently I'd pretty much given up on doing virtualization things entirely, but for various reasons beyond the scope of this post I've been inspired to begin tinkering with virtualization solutions again.
As a matter of course, I found myself trying KVM in a serious way for the first time. This experience both generated a new list of annoyances and reminded me about all the things I didn't like about Xen. I've collected these annoyances and thoughts into the following post. I hope that these thoughts will be helpful for people thinking about virtualization pragmatically, and also help identify some of the larger to pain points with the current solution.
Xen Hardships: It's all about the Kernel
Xen is, without a doubt, the more elegant solution from a design perspective and it has a history of being the more robust and usable tool. Performance is great, Xen hosts can have up-times in excess of a year or two.
The problem is that dom0 support has, for the past 2-3 years, been in shambles, and the situation isn't improving very rapidly. For years, the only way to run a Xen box was to use an ancient kernel with a set of patches that was frightening, or a more recent kernel with ancient patches forward ported. Or you could use cutting edge kernel builds, with reasonably unstable Xen support.
A mess in other words.
Now that Debian Squeeze (6.0) has a pv-ops dom0 kernel, things might look up, but other than that kernel (which I've not had any success with, but that may be me,) basically the only way to run Xen is to pay Citrix  or build your own kernel from scratch, again results will be mixed (particularly given the non-existent documentation,) maintenance costs are high, and a lot of energy will be duplicated.
What to do? Write documentation and work with the distributions so that if someone says "I want to try using Xen," they'll be able to get something that works.
KVM Struggles: It's all about the User Experience
The great thing about KVM is that it just works. "sudo modprobe kvm kvm-intel" is basically the only thing between most people and a KVM host. No reboot required. To be completely frank, the prospect of doing industrial-scale virtualization on-top of nothing but the Linux kernel and with a wild module in it, gives me the willies is inelegant as hell. For now, it's pretty much the best we have.
The problem is that it really only half works, which is to say that while you can have hypervisor functionality and a booted virtual machine, with a few commands, it's not incredibly functional in practical systems. There aren't really good management tools, and getting even basic networking configured off the bat, and qemu as the "front end" for KVM leaves me writhing in anger and frustration. 
Xen is also subject to these concerns, particularly around netowrking. At the same time, Xen's basic administrative tools make more sense, and domU's can be configured outside of interminable non-paradigmatic command line switches.
The core of this problem is that KVM isn't very Unix-like, and it's a problem that is rooted in it's core and pervades the entire tool, and it's probably rooted in the history of its development.
What to do? First, KVM does a wretched job of anticipating actual real-world use cases, and it needs to do better at that. For instances it sets up networking in a way that's pretty much only good for software testing and GUI interfaces but sticking the Kernel on the inside of the VM makes it horrible for Kernel testing. Sort out the use cases, and there ought to be associated tooling that makes common networking configurations easy.
Second, KVM needs to at least pretend to be Unix-like. I want config files with sane configurations, and I want otherwise mountable disk images that can be easily mounted by the host.
|||The commercial vendor behind Xen, under whose stewardship the project seems to have mostly stalled. And I suspect that the commercial distribution is Red Hat 5-based, which is pretty dead-end. Citrix doesn't seem to be very keen on using "open source," to generate a sales channel, and also seems somewhat hesitant to put energy into making Xen easier to run for existing Linux/Unix users.|
|||The libvirtd and Virt Manager works pretty well, though it's not particularly flexible, and it's not a simple command line interface and a configuration file system.|