Recently, I found out that there is the QEMU project. I've used VirtualBox before, and I know about Xen and VMWare.
What are the differences between QEMU and VirtualBox? Should I stick with VirtualBox?
In which cases is QEMU better?
Basically both have features which the other does not have, so this might ease the decision. QEMU/KVM is better integrated in Linux, has a smaller footprint and should therefore be faster.
VirtualBox is a virtualization software limited to x86 and amd64 architecture. Xen uses QEMU for the hardware assisted virtualization, but can also paravirtualize guests without hardware virtualisation. QEMU supports a wide range of hardware and can make use of the KVM when running a target architecture which is the same as the host architecture.
Xen is a Type-1 hypervisor where VirtualBox and QEMU are considered as Type-2 hypervisors (also there might be a debate considering kvm being a kernel module).