OpenVZ (by Parallels Inc.)
OpenVZ is is a Linux-based virtualization platform based on the Linux Kernel. OpenVZ allows a physical server to run multiple isolated operating system instances known as containers. OpenVZ can only run Linux-based operating systems such as CentOS, Fedora, Gentoo, and Debian.
One disadvantage of OpenVZ is that users are not able to make any kernel modifications. All virtual servers have to get along with the kernel version the host runs on. However because it doesn’t have the overhead of managing virtual machines, it is very fast and efficient compared to Xen, KVM and VMware.
OpenVZ is the open source version of Virtuozzo. It comes without the high-end features that accompany it’s expensive big brother, Virtuozzo. Being open source, it works with an open source VPS control panel such as SolusVM which keeps hosting charges low.
If you are looking for a fast, affordable and good VPS solution (memory, disk space, bandwidth, etc.) , OpenVZ is for you.
Virtuozzo (also by Parallels Inc. )
Efficient use of server resources is its primary strength. Virtuozzo partitions a Windows or Linux server into multiple execution environments (virtual environments). Using the OS virtualization method, Virtuozzo is noted for its performance and virtualization density (hundreds of virtual environments can run simultaneously).
However, since Virtuozzo is not open-source, it is not free and adds an additional cost to the hosting charges. It comes with its own VPS control panel.