Oracle VM – Virtualization Modes or Domain Types – Part 1

Introduction

Oracle VM introduced two main modes or domain types:

1. Paravirtualized (PVM):A virtual machine with a kernel that is recompiled to be made aware of the virtual environment. Runs at near native speed, with memory, disk and network access optimized for maximum performance.Paravirtualized guests use generic, idealized device drivers, which are part of the guest’s OS. The I/O operations using these generic device drivers are mapped to the real device drivers in dom0. The generic, abstracted drivers in the guest seldom change and provide excellent guest stability. The dom0 domain, alternatively, can use the native hardware vendor drivers, and the guests can safely migrate to another dom0 with slightly different drivers.

   For other resources such as CPU and memory, paravirtualized kernels make special “hypercalls” to the Xen hypervisor. These hypercalls provide better performance by reducing the number of instructions and context switches required to handle an incoming request. By contrast, on an emulated (hardware virtualized) guest, driver requests engage the guest’s interrupt handler, increasing the I/O operation overhead.

2. Hardware Virtualized Machine (HVM):A hardware virtualized guest runs on the virtualization platform as it would on a physical host. Because the device drivers of the hardware virtualized guest are emulated, dom0 must contain device emulation code to support the guest’s device drivers. The other types of privileged instructions issued by the hardware virtualized guest, for example, for CPU or memory access, are not emulated, but are trapped, which also requires support from CPU vendors.The guest’s OS does not require any modification to run as a hardware virtualized guest.

   A virtual machine with an unmodified guest operating system. It is not recompiled for the virtual environment. There may be substantial performance penalties running as a hardware virtualized guest. Enables Microsoft Windows™™ operating system to be run, as well as legacy operating systems. Hardware virtualization is only available on Intel® VT or AMD SVM CPUs.

There is third type that combined from two main modes:

• Hardware Virtualized Machine with Paravirtualized Drivers (PVHVM): This mode is identical to a hardware virtualized machine, but with additional paravirtualized drivers installed in the guest’s operating system to improve virtual machine performance.

When you are creating virtual machine, you should choose domain type but there is some limitations, restrictions and recommendation that we’ll review them in next part.

You can read more information on Oracle Help Center