（1）Binary writing
Binary writing has the nice benefit that it allows most of the virtual environment to run in userspace, but imposes a performance penalty.
The binary rewriting approach requires that the instruction stream be scanned by the virtualization environment and privileged instructions identified. These are then rewritten to point to their emulated versions.
Binary writing的核心之处在于把privileged instructions重写。
（2）Paravirtualization
Rather than dealing with problematic instructions, paravirtualization systems like Xen simply ignore them.
If a guest system executes an instruction that doesn’t trap while inside a paravirtualized environment, then the guest has to deal with the consequences. Conceptually, this is similar to the binary rewriting approach, except that the rewriting happens at compile time (or design time), rather than at runtime.
The environment presented to a Xen guest is not quite the same as that of a real x86 system. It is sufficiently similar, however, in that it is usually a fairly simple task to port an operating system to Xen.
From the perspective of an operating system, the biggest difference is that it runs in ring 1 on a Xen system, instead of ring 0. This means that it cannot perform any privileged instructions. In order to provide similar functionality, the hypervisor exposes a set of hypercalls that correspond to the instructions.
Paravirtualization核心之处在于hypervisor提供hypercalls给Guest OS，以弥补其不能使用privileged instructions。
（3）Hardware-Assisted Virtualization
Now, both Intel and AMD have added a set of instructions that makes virtualization considerably easier for x86. AMD introduced AMD-V, formerly known as Pacifica, whereas Intel’s extensions are known simply as (Intel) Virtualization Technology (IVT or VT). The idea behind these is to extend the x86 ISA to make up for the shortcomings in the existing instruction set. Conceptually, they can be thought of as adding a “ring -1” above ring 0, allowing the OS to stay where it expects to be and catching attempts to access the hardware directly. In implementation, more than one ring is added, but the important thing is that there is an extra privilege mode where a hypervisor can trap and emulate operations that would previously have silently failed.
IVT adds a new mode to the processor, called VMX. A hypervisor can run in VMX mode and be invisible to the operating system, running in ring 0. When the CPU is in VMX mode, it looks normal from the perspective of an unmodified OS. All instructions do what they would be expected to, from the perspective of the guest, and there are no unexpected failures as long as the hypervisor correctly performs the emulation.
A set of extra instructions is added that can be used by a process in VMX root mode. These instructions do things like allocating a memory page on which to store a full copy of the CPU state, start, and stop a VM. Finally, a set of bitmaps is defined indicating whether a particular interrupt, instruction, or exception should be passed to the virtual machine’s OS running in ring 0 or by the hypervisor running in VMX root mode.
Hardware-Assisted Virtualization（也称之为HVM，Hardware Virtual Machine），可以运行unmodified OS，其核心之处在于CPU层面提供了新的privilege mode和指令集来支持虚拟化。