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Classification of virtual machine monitors by architecture and implementation

Core Idea: Hypervisors are classified into distinct types based on their architecture, implementation, and relationship to hardware, with each type offering different performance characteristics, use cases, and management approaches.

Key Elements

Type 1 (Bare-Metal) Hypervisors

• Definition: Runs directly on host hardware without an underlying operating system
• Architecture:
  • Direct hardware access
  • Minimal software layer
  • Hardware resource control
  • Often includes a privileged management VM or domain
• Advantages:
  • Higher performance
  • Better resource control
  • Enhanced security
  • Lower overhead
  • More efficient resource utilization
• Examples:
  • Hyper-V (Microsoft)
  • ESXi (VMware)
  • Xen (open source, used by Citrix)
  • KVM (Linux kernel-based)
• Primary Use Cases:
  • Enterprise data centers
  • Cloud infrastructure
  • Production server environments

Type 2 (Hosted) Hypervisors

• Definition: Runs on top of a conventional operating system
• Architecture:
  • Depends on host OS for hardware access
  • Implemented as an application
  • Uses host OS drivers and services
• Advantages:
  • Easier to install and use
  • Better desktop integration
  • Can leverage host OS capabilities
  • More hardware compatibility through host OS
• Examples:
  • VMware Workstation/Fusion
  • Oracle VirtualBox
  • Parallels Desktop
  • QEMU
• Primary Use Cases:
  • Development and testing
  • Desktop virtualization
  • Education and training
  • Running multiple OS environments

Hybrid Hypervisors

• Definition: Combine aspects of both Type 1 and Type 2
• Architecture:
  • Often start as Type 2 but convert host to privileged VM
  • May run alongside the host OS rather than above it
• Examples:
  • Microsoft Hyper-V (loads before Windows but relies on parent partition)
  • Apple Hypervisor Framework
  • KVM (technically a hybrid as it turns Linux into a hypervisor)

Specialized Classifications

• Microkernel Hypervisors: Minimal code base for security and reliability
• Monolithic Hypervisors: Include device drivers and services in hypervisor layer
• Hardware-assisted Hypervisors: Rely on CPU virtualization extensions
• Software-only Hypervisors: Use binary translation without hardware assistance
• Paravirtualized Hypervisors: Require modified guest operating systems

Comparative Factors

• Performance overhead
• Memory management techniques
• I/O handling approaches
• Security isolation mechanisms
• Management interfaces
• Scalability characteristics

Additional Connections

• Broader Context: Virtualization Architecture (broader design approaches)
• Applications: Virtualization Platform Selection (decision criteria)
• See Also: Virtual Machine Monitor Evolution (historical development)

References

1. "Virtualization For Dummies" by Bernard Golden
2. "Computer Architecture: A Quantitative Approach" (relevant sections)

#virtualization #hypervisor #system-architecture #classification

Connections:

Sources:

• From: Hyper-V - Wikipedia