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I am going through the topic of virtualization and i am totally sucked up understanding the basic concept, Wikipedia does provide some relevant information, but it is not good enough for me to understand the basic idea. The concept will be of 2 to 3 line, but neither I am able to find them on net, nor on the book.

I will be pleased if someone gives me a basic understanding of these three types. I am well aware of virtualization and understand it well, but these 3 types...

mfs
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2 Answers2

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  • Paravirtualization is virtualization in which the guest operating system (the one being virtualized) is aware that it is a guest and accordingly has drivers that, instead of issuing hardware commands, simply issue commands directly to the host operating system. This also includes memory and thread management as well, which usually require unavailable privileged instructions in the processor.

  • Full Virtualization is virtualization in which the guest operating system is unaware that it is in a virtualized environment, and therefore hardware is virtualized by the host operating system so that the guest can issue commands to what it thinks is actual hardware, but really are just simulated hardware devices created by the host.

  • Hardware Assisted Virtualization is a type of Full Virtualization where the microprocessor architecture has special instructions to aid the virtualization of hardware. These instructions might allow a virtual context to be setup so that the guest can execute privileged instructions directly on the processor without affecting the host. Such a feature set is often called a Hypervisor. If said instructions do not exist, Full Virtualization is still possible, however it must be done via software techniques such as Dynamic Recompilation where the host recompiles on the fly privileged instructions in the guest to be able to run in a non-privileged way on the host.

  • There is also a combination of Para Virtualization and Full Virtualization called Hybrid Virtualization where parts of the guest operating system use paravirtualization for certain hardware drivers, and the host uses full virtualization for other features. This often produces superior performance on the guest without the need for the guest to be completely paravirtualized. An example of this: The guest uses full virtualization for privileged instructions in the kernel but paravirtualization for IO requests using a special driver in the guest. This way the guest operating system does not need to be fully paravirtualized, since this is sometimes not available, but can still enjoy some paravirtualized features by implementing special drivers for the guest.

wallyk
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Dougvj
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    In paravirtualisation, as you mention the guest is aware of and issues the commands to the Host OS compared to issuing the hardware commands to the pseudo hardware devices created by hypervisor in case of full-virtualization. I am puzzled as to How does paravirtualization happen in case of a type 1 hypervisor where there is no host OS. To whom does the guest issues the hardware commands to? – Gaurav Parashar May 05 '16 at 06:09
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    @gauravparashar It will issue the commands directly to the hypervisor as though it were a host OS. The Hypervisor has to at least have a basic paravirtualization API and in that sense it is a very basic host OS. – Dougvj May 06 '16 at 01:01
  • @Dougvj, Does Full Virtualization implies Hardware Assisted Virtualization? Do they go hand-in-hand? – Pacerier May 09 '16 at 16:04
  • @Pacerier No, you can still do full virtualization using Dynamic Recompilation (explained in the last sentence under Hardware Assisted Virtualization) – Dougvj May 09 '16 at 17:37
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    I like your definition of hardware virtualization. I looked everywhere but the other explanations were not clear enough for me. Thanks! – 10110 Apr 22 '19 at 00:15
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In the case of hardware assisted virtualisation, the virtualisation is designed in. Instruction set provides instructions for partitioning the host. See VT-x technology of Intel as an example. So that the hypervisor works directly with hardware without using any operating system to acces it and provide full virtualisation

manh chau nguyen
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