KVM (Kernel-based Virtual Machine) is, on the other hand a full virtualization solution for Linux containing virtualization extensions (Intel VT or AMD-V). Using KVM, one can have Linux and Windows virtual machines running side by side on the same hardware. Each virtual machine has private virtualized hardware: own kernel, a network card, disk, graphics adapter, etc. Since it has its own virtualized hardware, the virtual server will act completely on its own.
The pros with KVM is that all applications should work exactly as on a dedicated server, usability of its own kernel, the flexibility of migrating from virtual machine to dedicated server.
These perks comes with a price, the price of more resources needed for the host, meaning a small performance penalty regarding I/O and CPU.
OpenVZ (Open Virtuozzo) is an operating system-level virtualization technology based on the Linux kernel and operating system. OS level virtualization means many basic components exist once on the machine, and are used by all guests (like the identical kernel). You can only run the kernels that are supported by the provider. Burstable RAM is often portrayed as OpenVZ’s USP. It is touted as an advantage over the other virtualization technologies. This technology makes additional memory available to you when your website needs it. Even if OpenVZ has to kill a process critical for your setup or pull memory from an account which is not using its allocated memory at that time, it will. It’s great until the account that loaned its memory suddenly needs it, both may crash or experience slowdown while memory is moved around.