Virtuozzo creates multiple isolated Virtual Private Servers (VPSs) on a single physical server to share hardware, licenses and management effort with maximum efficiency. Each VPS performs and executes exactly like a stand-alone server for its users and applications as it can be rebooted independently and has its own root access, users, IP addresses, memory, processes, files, applications, system libraries, and configuration files. It is like XEN software. I guess it is growing competition from XEN and others force them to open source its kernel.
The new project is called OpenVZ. It is an Operating System-level server virtualization solution, built on Linux. OpenVZ creates isolated, secure virtual private servers (VPSs) or virtual environments on a single physical server enabling better server utilization and ensuring that applications do not conflict.