Edited sentence: All virtual servers have to be happy with the Linux kernel version that the host runs on.However because it doesn't have the overhead of a true hypervisor it is very fast an efficient.OpenVZ is not true virtualization but really containerization like BSD Jails.OpenVZ uses a common file system so each virtual environment is just a directory of files that is isolated using chroot.OpenVZ uses a single patched Linux kernel and therefore can run only Linux.So if a gig of ram is allocated but not used then it is available to others.Technologies like VMWare and XEN are more flexible in that they virtualize the entire machine and can run multiple operating systems.The advantages however are that memory allocation is soft in that memory not used in one virtual environment can be used by others or for disk caching.The obvious big disadvantage is the single kernel.This a virtual machine can be cloned by just copying the files in one directory to another and creating a config file for the virtual machine and starting it