Types of Virtualization

Virtualization Types

Amazon Machine Images use one of two types of virtualization: paravirtual (PV) or hardware virtual machine (HVM). Some current generation instance types support only HVM AMIs, while some previous generation instance types support only PV AMIs. The main difference between PV and HVM AMIs is the way in which they boot and whether or not they can take advantage of special hardware extensions (CPU, network, and storage) for better performance.
Paravirtual (PV)
Paravirtual AMIs boot with a special boot loader called PV-GRUB, which starts the boot cycle and then chain loads the kernel specified in themenu.lst file on your image. Only Linux AMIs can use PV virtualization. Paravirtual guests can run on host hardware that does not have explicit support for virtualization, but they cannot take advantage of special hardware extensions such as enhanced networking or GPU processing. Historically, PV guests had better performance than HVM guests in many cases, but because of enhancements in HVM virtualization and the availability of PV drivers for HVM AMIs, this is no longer true.
For more information on PV-GRUB and its use in Amazon EC2, see PV-GRUB.
Hardware Virtual Machine (HVM)
HVM AMIs are presented with a fully virtualized set of hardware and boot by executing the master boot record of the root block device of your image. Both Linux and Windows AMIs can use HVM virtualization. This virtualization type provides the ability to run an operating system directly on top of a virtual machine without any modification, as if it were run on the bare-metal hardware. The Amazon EC2 host system emulates some or all of the underlying hardware that is presented to the guest.
Unlike PV guests, HVM guests can take advantage of hardware extensions that provide fast access to the underlying hardware on the host system. For more information on CPU virtualization extensions available in Amazon EC2, see Server Virtualization on Intel's website. HVM AMIs are required to take advantage of enhanced networking and GPU processing. In order to pass through instructions to specialized network and GPU devices, the OS needs to be able to have access to the native hardware platform; HVM virtualization provides this access. For more information, see Enhanced Networking and GPU Instances.
PV on HVM
Paravirtual guests traditionally performed better with storage and network operations than HVM guests because they could leverage special drivers for I/O that avoided the overhead of emulating network and disk hardware, whereas HVM guests had to translate these instructions to emulated hardware. Now these PV drivers are available for HVM guests, so operating systems that cannot be ported to run in a paravirtualized environment (such as Windows) can still see performance advantages in storage and network I/O by using them. With these PV on HVM drivers, HVM guests can get the same, or better, performance than paravirtual guests.

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