OpenVZ is a Virtualization platform. It is based on a single Linux kernel which has been modified to support different Linux virtual environments. On the other hand Xen is a paravirtualization platform, just similiar to hardware emulation. Paravirtualization works by creating an interface between the virtual environment’s OS and the hardware, that queues and responds to the OS requests from OS modified to interact with the paravirtualization interface.
OpenVZ and Xen use different virtualization approaches: