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Hardware Virtual Machine vs Paravirtualization

3 types of virtualization:

  • Full Virtualization (HVM): guest OS thinks that it is running directly on the hardware; uses a hypervisor, which directly communicates with a physical server's disk space and CPU. Each virtual server is independent and unaware of the other virtual servers.
  • Para-Virtualization (PV): guest OS knows that it is running on a hypervisor instead of base hardware, recognizes that other virtual machines are running on the same machine; uses a hypervisor, each OS on the virtual servers is aware of one another.
  • OS-Level Virtualization (Containers): does not use a hypervisor. The virtualization capability is part of the physical server OS (e.g. cgroup).

Xen supports 2 virtualization types; Amazon supports 2 types as it runs on Xen.

PV

  • An OS or Kernel called Hypervisor is installed on the hardware.
  • Dom0 is called the "privileged domain" which can issue commands to the hypervisor.

Pros

  • Stability/Performance is close to the real servers and hardware virtualization.
  • Overhead is very low.

Cons

  • Implementation is tough.
  • Both the host & guest kernels has to be patched.
  • Supports Linux only.
  • can’t change the OS options during install.
  • Can’t compile and install a custom kernel.

HVM

  • Stands for Hardware-assisted virtual machine.
  • Provides complete hardware isolation. The hardware provides support to run independently for each OS.

Pros

  • Can run Linux and Windows.
  • Complete secure hardware isolation.
  • Resembles close to a physical server.
  • Greater stability.

Cons

  • Low performance, because of the overheads at the hardware level.