VirtualBox is currently my favorite virtualization software and
used it mainly for trying different Linux distributions, testing
codes, and running Windows as guest operating system on my Ubuntu
workstation. Some of its supported guest OS are BSDs, OS/2 Warp,
Windows, Solaris, Haiku, Syllable, ReactOS and SkyOS. VirtualBox
supports hardware virtualization for both the Intel VT-x and the
AMD AMD-V extensions. Hard disks are emulated in one of three disk
image formats: VirtualBox Virtual Disk Image (VDI); VMware Virtual
Machine Disk Format (VMDK); and Microsoft Virtual PC VHD format.
This means that a VirtualBox virtual machine can use disks that
were created in VMware or Microsoft Virtual PC aside from its own
native disk format.
Xen is a hypervisor that supports x86-64, Itanium, PowerPC 970, and
IA-32 architectures. It can run a number of guest operating systems
on a single computer hardware simultaneously. Xen utilizes a form
of virtualization known as paravirtualization, which means guests
run a modified operating system using a special hypercall ABI in
place of specific architectural features. Because of this, Xen can
achieve high performance even on x86 host architecture that is
known to have an issue with traditional virtualization
Based on the Linux kernel and operating system, OpenVZ is an
operating system-level virtualization technology."