Xen Architecture

 Xen Architecture

1. What is Xen?
• Definition: Xen is an open-source type-1 hypervisor that allows multiple operating systems to run simultaneously on the same hardware.
• Type-1 hypervisor means it runs directly on hardware (bare-metal), not on top of another OS.
• Main purpose: Server virtualization — used in cloud platforms like AWS.

2. Xen Architecture
Think of Xen as a traffic controller between hardware and operating systems.
Main Components:
1. Xen Hypervisor
• The core layer that runs directly on the CPU.
• Handles CPU scheduling, memory management, and I/O requests.
• Provides an abstraction layer between hardware and OS.
2. Domain 0 (Dom0)
• The first virtual machine started by Xen.
• Has special privileges to directly access hardware drivers.
• Manages other virtual machines (DomU).
• Runs a modified Linux OS with Xen management tools (xend, xl).
3. Domain U (DomU)
• User domains — guest operating systems.
• They do not have direct hardware access; they go through Dom0 for I/O.
• Can be:
• Paravirtualized (PV) — OS is modified to work with Xen.
• Hardware Virtual Machine (HVM) — uses CPU virtualization features, unmodified OS.
4. Control Interfaces
• Tools and APIs for creating, starting, stopping, and managing VMs.
• Example: xl create myvm.cfg
⸻
Xen Architecture Diagram (Exam-friendly)
   +------------------------------+
   |        Guest OS (DomU)        |
   +------------------------------+
   |        Guest OS (DomU)        |
   +------------------------------+
   | Privileged OS (Dom0) + Tools  |
   +------------------------------+
   |        Xen Hypervisor         |
   +------------------------------+
   |          Hardware             |
   +------------------------------+
⸻
3. Guest Operating System in Xen
A Guest OS is any operating system running inside a Xen virtual machine (DomU or Dom0).
Types of Guest OS in Xen:
1. Paravirtualized (PV) Guest
• OS is modified to work with Xen hypervisor calls.
• Direct access to Xen APIs for better performance.
• Example: Modified Linux kernel for Xen.
2. Full Virtualized / HVM Guest
• OS is not modified.
• Uses CPU features like Intel VT-x or AMD-V.
• Xen emulates hardware so the OS thinks it’s running on a real machine.
• Example: Windows running on Xen.
⸻
Guest OS Role
• Executes applications.
• Uses virtual hardware provided by Xen.
• Sends I/O requests (disk, network) through Dom0.
⸻
✅ Key Points for MSBTE Exam:
• Xen is Type-1 Hypervisor → runs directly on hardware.
• Dom0: First booted VM, has direct hardware control, manages DomU.
• DomU: Guest OS VMs, run in isolated environments.
• Guest OS can be PV (modified) or HVM (unmodified).
• Xen provides isolation, resource sharing, and security between VMs.