OpenVZ VS Xen, Vmware

The operating system level architecture(OpenVZ) has low overhead that helps to maximize efficient use of server resources. Due to a single-kernel approach, this type of virtualization introduces only a negligible overhead and allows running hundreds of virtual private servers on a single physical server. In contrast, approaches such as emulation (like VMware) and paravirtualization (like Xen or UML) cannot achieve such level of density, due to overhead of running multiple kernels. On the other hand, operating system-level virtualization does not allow running different operating systems (i.e. different kernels), although different libraries, distributions etc. are possible

Since there is a single OS kernel which maintains all the partitions, isolation and resource management become very important. Without proper isolation security can be compromised, and without proper resource management an application from one partition can abuse resources and thus cause a denial of service for other partitions. Resources controlled and limited can include: CPU time, disk space, I/O bandwidth, network access, and all the other finite resources like RAM, shared memory, locked pages, number of processes, socket buffers etc. For example, OpenVZ provide a set of more than 20 finite resources that are accounted and limited on a per-partition basis.
Compared to the hardware-assisted virtualization solutions (like IBM's LPAR), OS-level virtualization has the benefit of running on inexpensive commodity hardware

OS level virtualization solutions initially gained popularly with service providers who needed a very low overhead, customizable, cost effective solution to provide hosting services to customers. The technology is now becoming more widely deployed as many different industries and customers are recognizing the benefits and differences between OS virtualization and hardware virtualization. The common uses and scenarios now include server and OS consolidation, business continuity (disaster recovery and high availability) and centralized desktop virtualization.

An owner (root) of OpenVZ physical server (also known as Hardware Node) can see all the VE processes and files. That makes mass management scenarios possible. Consider that VMware or Xen is used for server consolidation: in order to apply a security update to your 10 virtual servers you have to log in into each one and run an update procedure – the same you would do with the ten real physical servers.
In OpenVZ case, you can run a simple shell script which will update all (or just some selected) VEs at once.

http://wiki.openvz.org/Introduction_to_virtualization

http://en.wikipedia.org/wiki/Operating_system-level_virtualization