Virtualisation: virtual machines and containers

Virtual machines

A virtual machine (VM) is a software emulation of a complete computer. A hypervisor sits on the physical hardware (or on a host OS) and shares out CPU, memory, storage and network between VMs.

• Each VM has its own full guest operating system, so a Linux VM and a Windows VM can run side by side on one server.
• VMs are strongly isolated: a crash or infection in one does not affect the others.
• VMs are large (gigabytes) and take time to boot because each loads a whole OS.

Containers

Containerisation packages an application with its libraries and settings into a container that runs on a container engine. Containers share the host operating system's kernel rather than each having a full OS.

• They are lightweight (megabytes), start in seconds and many more fit on one server.
• 'It works on my machine' problems disappear: the container runs the same anywhere.
• Isolation is weaker than a VM's because the kernel is shared, and every container must suit the host OS kernel.

Why virtualisation is needed

Benefits: better use of hardware (one server runs many workloads, saving cost, space and energy); sandboxed testing of software or malware; run software for other operating systems; quick backup, cloning and moving of whole machines; scale up quickly in the cloud.

Drawbacks: some performance overhead; one hardware failure takes down every VM on it; the hypervisor is a single point of attack; licensing and management can be complex.

Worked example

A software company must test its app on three operating systems and also run 200 small web services. Which virtualisation approach suits each?

1. Testing on three operating systems needs three different full OSs: use virtual machines.
2. 200 small web services on the same OS need to be lightweight and start quickly: use containers sharing the host kernel.

Answer: VMs for different operating systems; containers for many lightweight services.

Check yourself

1. What is a virtual machine?
   • A physical server in a data centre
   • A program that translates high-level code
   • A backup copy of a hard disk
   • A software emulation of a complete computer, running its own operating system on shared hardware
   Show answer

   A software emulation of a complete computer, running its own operating system on shared hardware. A VM behaves like a separate computer but is software on a host.

2. What does a hypervisor do?
   • Translates domain names into IP addresses
   • Creates and runs virtual machines and shares physical resources between them
   • Compresses files for storage
   • Detects transmission errors
   Show answer

   Creates and runs virtual machines and shares physical resources between them. The hypervisor allocates CPU, memory and I/O to each VM.

3. How does a container differ from a virtual machine?
   • A container always needs more memory than a VM
   • A container includes a complete guest operating system
   • A container can only run on physical hardware without an OS
   • A container shares the host operating system's kernel instead of running its own full OS
   Show answer

   A container shares the host operating system's kernel instead of running its own full OS. Sharing the kernel makes containers light and quick to start.

4. Why are virtual machines useful for testing suspicious software?
   • The software runs in an isolated sandbox, so the host and other systems are protected
   • VMs run software faster than real hardware
   • VMs cannot connect to networks
   • Viruses cannot run on virtual machines
   Show answer

   The software runs in an isolated sandbox, so the host and other systems are protected. Isolation lets you observe it safely and delete the VM afterwards.

5. Which is an advantage of containers over virtual machines?
   • They give stronger isolation
   • They can run a different OS kernel from the host
   • They start in seconds and many more fit on one server
   • They need a separate licence for each guest OS
   Show answer

   They start in seconds and many more fit on one server. No full OS to boot or store.

6. Which is a drawback of running many virtual machines on one physical server?
   • A hardware failure on that server stops every VM on it
   • The VMs cannot share the CPU
   • Each VM needs its own physical network cable
   • VMs cannot be backed up
   Show answer

   A hardware failure on that server stops every VM on it. All eggs in one basket unless VMs are replicated elsewhere.

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Sources

• Pearson Edexcel International Advanced Subsidiary and Advanced Level in Computer Science specification (2026), Issue 1
• Pearson Edexcel IAL Computer Science sample assessment materials (Unit 3 paper, resource booklet and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.