A hypervisor, also known as a virtual machine manager/monitor (VMM), is computer hardware platform virtualization software that allows several operating systems to share a single hardware host
Each operating system appears to have the host’s processor, memory, and resources to it. Instead, the hypervisor is controlling the host processor and resources, distributing what is needed to each operating system in turn and ensuring that the guest operating systems/virtual machines are unable to disrupt each other.
The term ‘hypervisor’ originated in IBM’s CP-370 reimplementation of CP-67 for the System/370, released in 1972 as VM/370.
The term ‘hypervisor call’ refers to the par virtualization interface, by which a guest operating system accesses services directly from the higher-level control program.
This is the same concept as making a supervisor call to the same level operating system.
Types of Hypervisor:
The hypervisor runs directly on the underlying host system. It is also known as “Native Hypervisor” or “Bare metal hypervisor”. It does not require any base server operating system. It has direct access to hardware resources. Examples of Type 1 hypervisors include VMware ESXi, Citrix XenServer and Microsoft Hyper-V hypervisor.
Such kind of hypervisors are very efficient because they have direct access to the physical hardware resources(like CPU, Memory, Network, Physical storage). This causes the empowerment the security because there is nothing any kind of the third-party resource so that attacker couldn’t compromise with anything.
One problem with Type-1 hypervisor is that they usually need a dedicated separate machine to perform its operation and to instruct different VMs and control the host hardware resources.
A Host operating system runs on the underlying host system. It is also known as ‘Hosted Hypervisor”. Such kind of hypervisors doesn’t run directly over the underlying hardware rather they run as an application in a Host system (physical machine). Basically, software installed on an operating system. Hypervisor asks the operating system to make hardware calls. Example of Type 2 hypervisor includes VMware Player or Parallels Desktop. Hosted hypervisors are often found on endpoints like PCs. The type-2 hypervisor is are very useful for engineers, security analyst (for checking malware, or malicious source code and newly developed applications).
Such kind of hypervisors allows quick and easy access to a guest Operating System alongside the host machine running. These hypervisors usually come with additional useful features for guest machine. Such tools enhance the coordination between the host machine and guest machine.
Here there is no direct access to the physical hardware resources so the efficiency of these hypervisors lags in performance as compared to the type-1 hypervisors, and potential security risks are also there an attacker can compromise the security weakness if there is access to the host operating system so he can also access the guest operating system.