The most popular virtualization techniques are currently based on three technologies

Full-VM or HVM (Hardware virtual machine) Containers Paravirtualization

HVM based solution

Although Full-VM/HVM based virtualization can be achieved using a variety of configurations, we will discuss what is known as the Hypervisor based approach. In this approach, the hypervisor, a very thin software layer runs on the bare hardware. It then manages several operating systems instances, scheduling the systems resources between them all. Using hardware virtualization assistance technologies like Intel VT or AMD-V, these hypervisors are able to run unmodified operating systems like many Linux distributions, and many versions of Windows. As far as the state of the art goes, the CPU is virtualized using the hardware, but various I/O devices are still emulated in software. Critical devices like the network interface and the disk slow down due to this. There is a penalty on performance.


Container based solution

Container based solutions do not suffer the overhead of multiple kernels and emulated I/O device. However you have to note that containers are not designed to run multiple operating systems. They provide an illusion of a complete system to each container. A container is just a set of processes. Since there is only one kernel and the processes run natively on the hardware, there is no performance penalty. On Linux for instance, you can run multiple distributions on the same kernel. Container also provide the ability to meet SLAs by featuring Quality of Service (QoS) settings for each VM. You can precisely specify how much memory and CPU scheduling priority a particular VM must be offered.


Paravirtualization

The problem of emulating the CPU has been mitigated by technologies like VT and AMD-V. There is however a need to emulate various devices. This is the cost of virtualizing unmodified operating systems. Paravirtualization is a unique technique where the guest operating system is "made aware" of a special platform. Between the hardware and the guest operating system is a thin layer, traditionally called a hypervisor. The guest operating systems requests the hypervisor if it needs to perform any privileged operation. This avoids the overhead of the traditional trap-and-emulate model, while providing raw performance. While the advantages are obvious, there is a need to modify the guest operating system specifically for it to be able to run on the hypervisor. Xen today, is the most popular paravirtualization system available, but the only mainstream OS running on it is Linux. There have been private ports of the Windows operating system, however.