Memory allocation with OpenVZ is soft in that memory not used in one virtual environment can be used by others or for disk caching. While old versions of OpenVZ used a common file system (where each virtual environment is just a directory of files that is isolated using chroot), current versions of OpenVZ allow each container to have its own file system.
All OpenVZ containers share the same architecture and kernel version. This can be a disadvantage in situations where guests require different kernel versions than that of the host. However, as it does not have the overhead of a true hypervisor, it is very fast and efficient.
OpenVZ is free open source software, available under GNU GPL.OpenVZ is a container-based virtualization for Linux
It includes core of Virtuozzo functionality, like kernel and command line management tools for creating and managing containers.
Most applications running on a server can easily share a machine with others, if they could be isolated and secured. Further, in most situations, different operating systems are not required on the same server, merely multiple instances of a single operating system. OS-level virtualization systems have been designed to provide the required isolation and security to run multiple applications or copies of the same OS (but different distributions of the OS) on the same server.
Virtualization and isolation
Each container is a separate entity, and behaves largely as a physical server would. Each has its own:
System libraries, applications, virtualized /proc and /sys, virtualized locks, etc.
Users and groups
Each container has its own root user, as well as other users and groups.
A container only sees its own processes (starting from init). PIDs are virtualized, so that the init PID is 1 as it should be.
Virtual network device, which allows a container to have its own IP addresses, as well as a set of netfilter (iptables), and routing rules.
If needed, any container can be granted access to real devices like network interfaces, serial ports, disk partitions, etc.
Shared memory, semaphores, messages.