Difference between ESX,KVM and QEMU

Question

What are the major differences between ESX, KVM and QEMU?

Answer 1

Qemu is an open source user-space virtual machine monitor. It uses binary translation to run the guest instructions.

Pros : 
1) Has support of helpers which help a lot in debugging
2) Can be used to run guest of a different ISA. (You can emulate an ARM guest on x86 desktop)
3) Does not need hardware support
4) code is available. you can modify it for debugging

Cons:

1) Slow

KVM is an open source virtual machine monitor which is implemented in the linux kernel. Qemu can run a guest on KVM using the /dev/kvm interface.

Pros:
1) very fast
2) code is available in the kernel tree. you can modify it for debugging

Cons:
1) needs hardware support
2) Requires a user level software for interfacing (generally qemu)

VMX is a virtual machine monitor by VMWare. The source code of VMX is not freely available. But it supports both user space as well as hardware supported emulation.

Pros : 
1) Very easy to use. GUI is good.
2) Fast and configurable. 

Cons :
1) Cannot modify it to obtain more information about the guest.