Emulation of the underlying raw hardware
Emulation of the underlying raw hardware (native execution)
This approach is described as full virtualization of the hardware, and can be implemented using a Type 1 or Type 2 hypervisor. (A Type 1 hypervisor runs directly on the hardware; a Type 2 hypervisor runs on another operating system, such as Linux.) Each virtual machine can run any operating system supported by the underlying hardware. Users can thus run two or more different "guest" operating systems simultaneously, in separate "private" virtual computers.
The pioneer system using this concept was IBM's CP-40, the first (1967) version of IBM's CP/CMS (1967-1972) and the precursor to IBM's VM family (1972-present). With the VM architecture, most users run a relatively simple interactive computing single-user operating system, CMS, as a "guest" on top of the VM control program (VM-CP). This approach kept the CMS design simple, as if it were running alone; the control program quietly provides multitasking and resource management services "behind the scenes". In addition to CMS, VM users can run any of the other IBM operating systems, such as MVS or z/OS. z/VM is the current version of VM, and is used to support hundreds or thousands of virtual machines on a given mainframe. Some installations use Linux for zSeries to run Web servers, where Linux runs as the operating system within many virtual machines.
Full virtualization is particularly helpful in operating system development, when experimental new code can be run at the same time as older, more stable, versions, each in separate virtual machines. (The process can even be recursive: IBM debugged new versions of its virtual machine operating system, VM, in a virtual machine running under an older version of VM, and even used this technique to simulate new hardware.)
The standard x86 processor architecture as used in modern PCs does not actually meet the Popek and Goldberg virtualization requirements. Notably, there is no execution mode where all sensitive machine instructions always trap, which would allow per-instruction virtualization.
Despite these limitations, several software packages have managed to provide virtualization on the x86 architecture, even though dynamic recompilation of privileged code, as first implemented by VMware, incurs some performance overhead as compared to a VM running on a natively virtualizable architecture such as the IBM System/370 or Motorola MC68020. By now, several other software packages such as Virtual PC, VirtualBox, Parallels Workstation and Virtual Iron manage to implement virtualization on x86 hardware.
On the other hand, plex86 can run only Linux under Linux using a specific patched kernel. It does not emulate a processor, but uses bochs for emulation of motherboard devices.
Intel and AMD have introduced features to their x86 processors to enable virtualization in hardware.
Emulation of a non-native system
Virtual machines can also perform the role of an emulator, allowing software applications and operating systems written for another computer processor architecture to be run.
Some virtual machines emulate hardware that only exists as a detailed specification. For example:
* One of the first was the p-code machine specification, which allowed programmers to write Pascal programs that would run on any computer running virtual machine software that correctly implemented the specification.
* The specification of the Java virtual machine.
* The Common Language Infrastructure virtual machine at the heart of the Microsoft .NET initiative.
* Open Firmware allows plug-in hardware to include boot-time diagnostics, configuration code, and device drivers that will run on any kind of CPU.
This technique allows diverse computers to run any software written to that specification; only the virtual machine software itself must be written separately for each type of computer on which it runs.
Operating system-level virtualization
Operating System-level Virtualization is a server virtualization technology which virtualizes servers on an operating system (kernel) layer. It can be thought of as partitioning: a single physical server is sliced into multiple small partitions (otherwise called virtual environments (VE), virtual private servers (VPS), guests, zones, etc); each such partition looks and feels like a real server, from the point of view of its users. One example here is Solaris zones. You can have multiple guest OS running under the same OS (This is allowed on Solaris 10). But All guest OS have to use the same kernel level--you cannot run a different version; And also you cannot run a different OS than Solaris. Also AIX provides the same technique called Micro Partitioning
The operating system level architecture has low overhead that helps to maximize efficient use of server resources. The virtualization introduces only a negligible overhead and allows running hundreds of virtual private servers on a single physical server. In contrast, approaches such as virtualisation (like VMware) and paravirtualization (like Xen or UML) cannot achieve such level of density, due to overhead of running multiple kernels. From the other side, operating system-level virtualization does not allow running different operating systems (i.e. different kernels), although different libraries, distributions etc. are possible.