Types of hypervisors in Linux

What is a hypervisor?

A hypervisor, also known as a Virtual Machine Monitor (VMM), is a software virtualization layer that allows you to create and run multiple virtual machines (VMs) on a single server, as well as run different operating systems in isolation.

The physical server running the hypervisor is called the host machine, while each individual VM is called a guest machine.

The term “hypervisor” was coined in the 1970s based on the concept of a supervisor in an operating system kernel. By adding the prefix “hyper-”, it is considered the supervisor of the supervisors.

Hypervisors separate the resources of a VM from the physical hardware and allocate them appropriately, facilitating cloud migration and optimizing costs, accessibility, and scalability.

Benefits of hypervisors

Using hypervisors provides numerous advantages:

• Efficiency: allows more efficient use of physical servers, reducing costs.
• Flexibility: VM resources are abstracted from hardware, facilitating portability and workload distribution.
• Cloud scalability: supports multiple VMs on bare-metal servers or multi-tenant environments.
• Security and isolation: enhances security by separating virtual environments from the host system.

Types of hypervisors

According to Gerald J. Popek and Robert P. Goldberg, there are two main types of hypervisors: type 1 and type 2. Although the distinction is not always completely clear, it is useful for understanding their functionality and use.

Type 1 Hypervisors (bare-metal)

Type 1 hypervisors, also called bare-metal or native hypervisors, run directly on the host hardware and manage guest operating systems without the need for an intermediate OS.

• Functionality: interact directly with hardware, assigning resources to VMs and sharing them as needed.
• Typical use: data centers, web servers, and high-performance enterprise computing environments.
• Advantages: higher performance, efficiency, and isolation; less vulnerable to attacks because they are separated from the host OS.

Examples of Type 1 Hypervisors

• KVM (Kernel-based Virtual Machine): integrated into the Linux kernel, supports full virtualization and containers.
• VMware ESXi: enterprise bare-metal hypervisor, part of VMware vSphere.
• Microsoft Hyper-V: hypervisor integrated into Windows Server.
• Proxmox VE: Debian-based open-source virtualization platform, supports LXC and KVM.
• XEN: open-source hypervisor originally developed at Cambridge University, maintained by the Linux Foundation.
• Citrix Hypervisor: based on XEN, designed for enterprise environments.
• Red Hat Virtualization (RHV): KVM-based, enterprise-focused platform.
• OpenStack: open-source cloud platform that uses type 1 hypervisors for deploying VMs in public or private clouds.

Type 2 Hypervisors (hosted)

Type 2 hypervisors, also called hosted hypervisors, run on top of a conventional operating system as an application. They abstract guest OSes from the host OS.

• Functionality: request resources from the host OS to run VMs.
• Typical use: desktop environments, testing, development, or individuals who need to run multiple OSes simultaneously.
• Advantages: easy to install and manage, no advanced system knowledge required.
• Limitations: lower performance and isolation compared to type 1 hypervisors.

Examples of Type 2 Hypervisors

• VirtualBox: open-source hypervisor for Windows, Linux, and macOS.
• QEMU: open-source emulator and virtualizer.
• VMware Workstation Player and VMware Fusion: commercial hypervisors for PCs and Macs.
• Parallels Desktop: allows running Windows on macOS without rebooting.

Key Differences Between Type 1 and Type 2 Hypervisors

Conclusion

Hypervisors are the backbone of modern virtualization, enabling multiple virtual machines to run efficiently and securely. Choosing between a type 1 or type 2 hypervisor depends on the usage environment, available resources, and performance and security goals.

In enterprise and cloud environments, type 1 hypervisors are preferred for their high performance and isolation. For development, testing, or personal use, type 2 hypervisors are ideal due to their ease of use and flexibility.