The recent bruhaha surrounding Xen on LKML (http://lkml.org/lkml/2009/6/2/475) is really disheartening. Essentially, the Linux kernel devs are at a disconnect with users. Some are proposing narrow-minded ideas such as DROPPING software paravirt or merging Xen as a whole into the kernel.
I use Xen for a few primary reasons: it bar none has the best speed -- full software paravirtualization pays dividends here; it is mature; it works on perfectly good machines that don't happen to have the latest chips; it does hardware passthrough on these same systems; it has great live migration that actually works.
Ingo Molnar wants you to send all your perfectly good enterprise iron to the landfill even though these systems will last 10+ useful years without boneheaded software decisions such as this.
These same FUDsters want to strip the crossplatform nature of Xen dom0 out too. Xen dom0 runs on NetBSD and Solaris. It is a true hypervisor and will plug into exisiting architectures, and not force you to use Linux for everything.
I have to admire all the hoops Jeremy Fitzhardinge has jumped through to date, as I know my patience is wearing thin.
Xen powers huge sites such as Amazon and services like linode.com/slicehost.com. By not having dom0 in the kernel where distros such as Ubuntu and Fedora can easily integrate it, kernel devs are doing a disservice to users.
I use KVM, VMWare, and Virtual Box at work in addition, but Xen is firmly entrenched in my toolbox. The roadmap they have looks great, and I just don't see a reason for decline in Xen popularity. High availability in Xen 4.0 is what I've always been waiting for.
Jeremy has gone to great lengths to work with upstream but keeps getting shot down and asked to do something else when he meets one requirement. The solution is to merge Jeremy's conservative dom0 patch set and work on a technical solution to the patches that the FUDsters consider bad. It's what the users want!