[CentOS] Server Virtualization

Hello,

I work for a school in a New Zealand university and we are wanting to 
implement Server Virtualization for both CentOS and Windows systems.  So 
I thought I would ask here what experience people have had with this and 
what issues that you all think should be considered?

 From my own research it seems that VMWare or Xen are really the two 
major products to be considered, are there any others I should be 
considering ?

Is anyone running Linux "Guest" O/S's inside a Windows host ??  And if 
so can you share your reasons for this?

Anyway thank you for your time and any experiences / knowledge you are 
willing to share :)