KVM & Qemu

QEMU is a powerful emulator, which means that it can emulate a variety of processor types.

Xen uses QEMU for HVM guests, more specifically for the HVM guest's device model. The Xen-specific QEMU is called qemu-dm (short for QEMU device model)

QEMU uses emulation; KVM uses processor extensions (intel-VT) for virtualization.

Both Xen and KVM merge their various functionality to upstream QEMU, that way upstream QEMU can be used directly to accomplish Xen device model emulation, etc.

Xen is unique in that it has paravirtualized guests that don't require hardware virtualization.

Both Xen and KVM have paravirtualized device drivers that can run on top of the HVM guests.

The QEMU hypervisor is very similar to the KVM hypervisor. Both are controlled through libvirt, both support the same feature set, and all virtual machine images that are compatible with KVM are also compatible with QEMU. The main difference is that QEMU does not support native virtualization. Consequently, QEMU has worse performance than KVM and is a poor choice for a production deployment.
The typical uses cases for QEMU are

    Running on older hardware that lacks virtualization support.
    Running the Compute service inside of a virtual machine for development or testing purposes, where the hypervisor does not support native virtualization for guests.

One difference between them is that QEMU runs on a processor without needing for hardware virtualization extension(Intel VT/VT-d, AMD-V) while KVM uses it. Hardware virtualization extensions lets you acces the hardware on the physical machine directly. The downside is that KVM codebase can't emulate another architecture.
But when KVM is run on a machine without any HW virt ext., it switches back to QEMU to run the VM.

KVM is a type 1 hypervisor and Qemu is a Type 2 hypervisor. Type 1 hypervisor comes installed with the hardware system like KVM in Linux. KVM provides hardware acceleration for virtual machines but it need Qemu to emulate any operating system.

Qemu is a Type 2 hypverisor, it can be installed on an operating system and it runs as an indepent process and the instructions we give in Quemu will be executed on the host machine. Qemu can run independently without KVM as its a emulator however the performance will be poor as Qemu doesnt do any hardware acceleration.

KVM and QEMU – understanding hardware acceleration

To understand hardware acceleration, we must understand how Virtual Machine CPU works. In real hardware, the Operating System (OS) translates programs into instructions that are executed by the physical CPU. In a virtual machine, the same thing happens. However, the key difference is that the Virtual CPU is actually emulated (or virtualized) by the hypervisor. Therefore, the hypervisor software has to translate the instructions meant for the Virtual CPU and convert it into instructions for the physical CPU. This translation has a big performance overhead.

To minimize this performance overhead, modern processors support virtualization extensions. Intel support a technology called VT-x and the AMD equivalent is AMD-V. Using these technologies, a slice of physical CPU can be directly mapped to the Virtual CPU. Hence the instructions meant for the Virtual CPU can be directly executed the physical CPU slice.

KVM is the Linux kernel module that enables this mapping of physical CPU to Virtual CPU. This mapping provides the hardware acceleration for Virtual Machine and boosts its performance. Moreover, QEMU uses this acceleration when Virt Type is chosen as KVM.

Then what is TCG? If your server CPU does not support virtualization extension, then it is the job of the emulator (or hypervisor) to execute the Virtual CPU instruction using translation. QEMU uses TCG or Tiny Code Generator to optimally translate and execute the Virtual CPU instructions on the physical CPU.
KVM and QEMU – Type 1 or Type 2 hypervisor

The web pages of KVM and QEMU clearly show that KVM needs QEMU to provide full hypervisor functionality. By itself, KVM is more of a virtualization infrastructure provider.

QEMU by itself is a Type-2 hypervisor. It intercepts the instructions meant for Virtual CPU and uses the host operating system to get those instructions executed on the physical CPU. When QEMU uses KVM for hardware acceleration, the combination becomes a Type-1 hypervisor.

KVM and QEMU – the x86 dependency

Since KVM is really a driver for the physical CPU capabilities, it is very tightly associated with the CPU architecture (the x86 architecture). This means that the benefits of hardware acceleration will be available only if the Virtual Machine CPU also uses the same architecture (x86).

If a VM needs to run Power PC CPU but the hypervisor server has an Intel CPU, then KVM will not work. You must use QEMU as the Virt Type and live with the performance overhead.

KVM and QEMU – the conclusion

Based on the discussion above, it is quite clear that QEMU plays a very critical role in Linux based Open Source virtualization solutions. For all practical applications, QEMU needs KVM’s performance boost. However, it is clear that KVM by itself cannot provide the complete virtualization solution. It needs QEMU.

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