para virtualization
Paravirtualization is an implementation variant of server virtualization. In contrast to the full virtualization approach, where unmodified guest operating systems access virtual hardware, paravirtualized guests interact directly with the shared hardware via an application programming interface( API) provided by the virtualization layer - managed and controlled by the hypervisor.
The paravirtualization approach does not hide virtualization from the guest; instead, the virtualized operating system "knows" that it is running virtualized and must communicate with the hypervisor through an abstracted hardware interface. This requires the guest operating system to be adapted, which means that mostly only open source operating systems such as Linux and FreeBSD are used in the virtual machines. In most cases, only the kernel of the operating system is adapted.
The host system usually consists only of the hypervisor component and a privileged operating system for management purposes. Only the device drivers of this privileged operating system are used (with exceptions), which relieves the hypervisor of the difficult task of having to provide drivers for all guest operating systems itself.
Examples of products based on the paravirtualization model are Xen , SUN Logical Domains and IBM z/ VM.
In the course of the use of central processing units( CPU) with virtualization support (Intel VT, AMD-V), systems such as Xen extend the paravirtualization approach to unmodified closed-source operating systems such as Microsoft Windows: in this case, specially prepared network and hard disk drivers are installed if required, which are modified so that they interact with the hardware via the hypercall API of the Xen hypervisor (and not directly).
In the Linux kernel since version 2.6.20 there is a standardized API for paravirtualization with paravirt_ops, which can be used by VMware 's Virtual Machine Interface (VMI), for example. Thus, virtual machines adapted for various paravirtualization systems can theoretically run directly on the standard Linux kernel.