Q. I was reading this article and wanted to try out KVM, XEN, VMWare and VirtualBox simultaneously on my desktop system with 2GB RAM? Why I'm not able to use run multiple virtualization technologies simultaneously under Ubuntu Linux? Can a powerful Linux server run multiple virtualization technologies simultaneously?
A. Short answer - No.
Each virtualization software like XEN or KVM make changes to kernel so it is not possible to use them simultaneously.
Also, virtualization software needs full control over certain hardware resources such as CPU, memory, GPU and other aspects. Two software cannot control and share resources using virtualization technology.
You may experience serious system corruption or reboots when you try to access or share same hardware using various virtualization technologies. My recommendation only install and use one virtualization technology at a time.