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The modification of the source code of an operating system in order to run as a guest operating system in a specific virtual machine environment. Calls to the hardware from the guest OS are replaced with calls to the virtual machine monitor (VMM). For example, several operating systems, such as Linux, OpenBSD, FreeBSD and OpenSolaris, have been paravirtualized for the Xen virtual machine monitor.

Paravirtualization Vs. Emulation
The guest OS can run as is without modification if the VMM emulates the hardware. In this case, the calls from the guest OS drivers to the hardware are intercepted and managed by the VMM, which redirects them to the real drivers. In addition, calls from the guest OS to the virtual memory page tables are intercepted and managed by the VMM. Emulation enables any guest OS to run intact, but emulation is slower than if the guest OS were paravirtualized.

Paravirtualization may be an option. If support for virtual machines is present in the CPU hardware, the guest OS may not need modification. For example, prior to the virtualization circuits built into x86 CPUs, the Xen VMM required the guest OS to be modified. If Xen runs in later Intel VT or AMD-V CPUs, a guest OS can run as is. See virtual machine monitor, virtual machine, hardware virtualization and Xen.

Emulated Hardware
Just like running in a non-virtualized computer, a non-paravirtualized guest OS communicates with the hardware as usual. The VMM presents a "device model" to the guest OS, which emulates the hardware. In these illustrations and the one following, the emphasis is on the device drivers. Paravirtualization also refers to modifying the calls to the virtual memory tables.

Paravirtualized Guests
In a paravirtualized OS, the drivers are replaced with calls to the VMM interface. This example shows one VMM model. For more VMM architectures, see virtual machine monitor.
References in periodicals archive ?
the global leader in software for industry-standard virtualized desktops and servers, today announced that it is supporting cross-platform paravirtualization with the open-interface standard paravirt-ops in VMware Workstation 6, the sixth generation of its desktop virtualization software product.
VMware Workstation 6 Extends its Technology Leadership with Groundbreaking Record/Replay Capabilities, Numerous Industry-First Features and Paravirtualization Support
Customers are increasingly looking to the technology industry to provide solutions optimized for virtualization, a trend that was reinforced in 2006 with the emergence of hardware-assisted virtualization, paravirtualization in the operating system, broad ISV support for virtual machines and a rapidly growing ecosystem of service providers.
Although Moose Logic had worked with VMware before, Buyken decided XenSource was a better fit because of its ability to run Windows using paravirtualization, a method of virtualization that provides greater security and reduced overhead.
VMware has invested in numerous open technology efforts with the Linux community, including introducing the Virtual Machine Interface (VMI) paravirtualization specification, contributing to the paravirt-ops platform-independent paravirtualization API and opening up VMware's Virtual Machine Disk (VMDK) format for all developers to use freely.
Enhanced 64 bit hypervisor - Extends Xen's paravirtualization support to allow simultaneous virtualization of both 32- and 64-bit paravirtualized Linux virtual machines.
Sun will continue to add breakthrough virtualization technology to Solaris 10 through 2007 -- most notably with the planned addition of the open source Xen hypervisor, a paravirtualization technology that presents a software interface to virtual machines.
The company considered VMware but found that XenSource's paravirtualization technology provided superior performance at an unbeatable price-point.
Customers using XenEnterprise can leverage paravirtualization, which is the key innovation of the Xen hypervisor, together with microprocessor support for virtualization, to deliver near "bare metal" performance to virtualized guest operating systems.