Virtualbox, VMware, KVM or other for Ubuntu virtualization?

Virtualbox will be much easier for use in the home. The GUI is great and its very simple to use... not to mention the amount of users and documentation out there. And with the recent hardware 3D acceleration addition its a great choice.

Use Virtualbox.

For testing and virtualized XP for daily use I recommend VirtualBox. It's free and has all the necessary features to run desktop OS's. It is also cross platform which means you can move your virtual machines around.

For server type scenarios and always on machines I would use VMWare Server. It makes the management of machines simpler and easier.

I remember problems with VMWare on ubuntu. The new version has client not as standalone application, but as plugin to firefox - it didn't work for me. VirtualBox on the other hand is very easy and seems "nicer". VMWare however has ability of managing virtual machines by network (firefox), while VirtualBox you can manage only from machine it is running on.