In my opinion, Virtuozzo is faster than Xen, as Xen is para-virtualization and has its own kernel for every VPS, on the other hand Virtuozzo has only one single kernel. Although I can't provide any real data now, I think Virtuozzo just should be faster.
I think you should consider Xen, because its popularity today increasing due to the maturity which has more than just third party vendor support for control panels. However, choice is yours..
Well why don't you consider VMware. Look I am complete novice user of Linux, but I've purchased Linux server and learned how to use SSH. I understood a guide of VM installation and within a few hours everything is ready. According to my knowledge, VM provides great server console to operate Windows platform system, but an expert Linux administrator will take only several minutes to understand VM and can get everything working..
I have heard lot of things about Xen and Virtuozzo, but VM provides very professional documents and support forum is just an extra icing cream on cake, so it's pretty easy to understand. As I don't have any experience with UML so not confident about that platform.
Yes, the best virtualization platform is VMware and I am looking forward to it. IMO, the other options just don't come close to VM. Xen is also interesting platform with its para virtualization. I think its hypervisor level handles the redirection of inside operations rather than sending them straight to the CPU.
As for now we are only using Xen for our clients, and OpenVZ platform to partition some of our own website. However, some one told me that KVM is also worth and it is supported by solusVM.
I am also looking forward to KVM in the nearest future. The fact is SolusVM panel supports it now, which seems a big bonus. However, this is pretty exciting to know as somebody point out the better performance of KVM compared with other platforms..
If you are looking for a free virtualization application, you should choose OpenVZ. If you can pay some money for the virtualization application, you can choose Virtuozzo. If you are ready to pay a lot of money but need the best virtualization application, you can choose VMWare.