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What is hypervisor?

    A virtualization platform.

    － Enables you to run multiple operating systems on host computer(註1).

    － Provide isolated execution environments for each virtual machine.

    － Manage access between the guest operating systems(註2) running in virtual machines and the

        underlying hardware resources on host computer.

    註1：host computer 是指用來運行Hypervisor的電腦。   

    註2：guest operating systems 是指在Hypervisor上面運作的作業系統。

 

What is Type1 hypervisor?

    Type 1 hypervisors run directly on bare metal instead of within an operating system environment.

    － Provide the best performance, availability, and security of any form of hypervisor.

    － Example：Microsoft Hyper-V、Citrix XenServer、VMware ESX Server.

    － 從下圖就可以很清楚的看出，

         Type 1 hypervisors是直接執行在電腦硬體上，

         而不是在實體電腦的作業系統中，

         使得它的效能、可用性及安全性都會比較好。

 

What is Type2 hypervisor?

    Type 2 hypervisors run within an operating system environment running on the host computer.

    － Typically referred to as hosted virtualization.

    － Limits the number of virtual machines you can realistically run.

    － Example：Microsoft Virtual Server、VMware Server.

    － 從下圖就可以很清楚觀察到Type 1 hypervisors和Type 2 hypervisors的差別，

        Type 2 hypervisors是執行在作業系統環境中，

        缺點是由實體主機的作業系統管理所有虛擬機器中的運作，

        相對的，運作速度會比較慢，但是很方便，比較沒有相容性的問題。

 

其中Type 1 hypervisors又可根據其搭配的核心的不同，

分為Monolithic hypervisor或Microkernel hypervisors。

 

What is Monolithic Hypervisor?

    Monolithic hypervisors design involves using hypervisor-aware device drivers that are hosted within and

    managed.

    － Do not need a controlling, or parent, operating system.(benefit)

    － Device drivers must be specifically developed for the hypervisor.(drawback)

    － Example：VMware ESX Server.

    － 從上述可知，Monolithic hypervisor讓所有虛擬機器透過hypervisor-aware的驅動程式直接存取與目標硬

        體。

        這樣的優點是不需要控制用的作業系統，

        但缺點是此hypervisor-aware的驅動程式是被包含在核心中，

        彈性不足，對於硬體的限制很多，

        必須依據原廠指定的機種或是配備，才能成功地安裝與啟用。

 

What is Microkernel Hypervisors?

    Microkernel hypervisors have an operating system acting as the root, or parent, partition.

    － Do not require hypervisor-aware device drivers.

    － The parent partition provides the execution environment needed for device drivers to access

        the underlying physical hardware of the host computer.

    － Install device drivers only for physical devices in the operating system running in the parent

        partition.

    － The guest operating systems do not have direct access to the underlying hardware.

    － The guest operating systems can access physical devices only by communicating with the

        parent partition.

    － Example：Microsoft Hyper-V.

    － 從上述可知，Microkernel hypervisors和Monolithic hypervisor的差別在於

        不需要hypervisor-aware的驅動程式，

        現有的驅動程式可以立即被使用於虛擬化環境中，

        使得Hypervisor更小、更穩定，耗損也較小。

 

參考資料：

Understanding Microsoft Virtualization Solutions, by Mitch Tulloch with the Microsoft Virtualization Teams.