What is a Virtual Machine?
A virtual machine (VM) is a fully isolated software system that emulates the hardware of a physical computer. Each VM operates independently, running its own operating system and processes, with virtualized access to CPU, memory, disk, and network interfaces. This virtualization is made possible through a hypervisor, which abstracts physical hardware and allocates compute resources to multiple VMs on a single host. The most widely used open-source hypervisor for Linux environments is KVM (Kernel-based Virtual Machine). Integrated directly into the Linux kernel, KVM allows the host OS to function as a bare-metal hypervisor. When paired with QEMU or management layers like libvirt, KVM provides high-performance virtualization with strong isolation, making it a standard in enterprise, private cloud, and hybrid environments.
How Virtual Machines Are Constructed
Before a VM is powered on, it exists as a collection of files that define its identity and behavior. These files serve as the blueprint for the virtual hardware and operating system environment.
Virtual Disk Images
Every VM requires a virtual disk file that acts as its primary storage device. This file contains the guest OS, applications, and user data. Two common formats are QCOW2 and RAW. QCOW2 supports copy-on-write layering, compression, and snapshots, which makes it suitable for agile development and testing environments. RAW is preferred when performance is the top priority, as it removes format overhead and reads more directly from disk.
Configuration Files
VM specifications are stored in structured files (typically XML or JSON) used by the hypervisor to define the virtual hardware. These include settings for CPU cores, memory allocation, disk paths, network interfaces, and device passthrough options. Configuration files allow the hypervisor to launch the VM with consistent parameters each time.
Snapshots and Backups
A snapshot captures the state of a VM at a specific point, including memory, disk contents, and runtime data. Snapshots are used to preserve known-good states, facilitate quick rollback, or support testing scenarios where reversibility is important. Snapshots may be stored as separate files or embedded within QCOW2 layers.
System Logs and Metadata
During runtime, the hypervisor generates logs that document VM activity. These include messages related to boot sequences, hardware emulation, shutdown events, and errors. Metadata files may track unique VM IDs, network configurations, or resource quotas, especially in managed or orchestrated environments.
What Happens at Boot Time
When a virtual machine is started, it goes through a process similar to physical hardware. The hypervisor maps the defined resources to virtual hardware interfaces, presents them to the guest OS, and launches the emulation layer. The VM’s operating system begins by loading its kernel into a virtualized CPU context. Using hardware acceleration (such as Intel VT-x or AMD-V), the hypervisor enables near-native performance by forwarding CPU instructions to the physical processor with minimal overhead.
After the kernel initializes, the operating system begins its init system (such as systemd) to start services, mount volumes, and load daemons required by applications. All resource access (memory, I/O, disk, and network) flows through virtual devices exposed by the hypervisor. Memory is sandboxed from the host and other VMs, and storage is mounted as if it were a local block device. Network traffic is passed through virtual interfaces that connect to the host’s bridge or switching layer. From the guest OS perspective, it behaves as though it’s running on dedicated hardware.
VMs Compared to Containers
While both virtual machines and containers enable workload isolation, their architectures differ significantly. VMs include a complete operating system and kernel, which provides strong separation from the host and from other VMs. This makes them ideal for situations that require high levels of security, compliance, or system-level customization. VMs can run different OS types on the same host, including combinations of Linux, Windows, or BSD.
Containers, in contrast, share the host’s kernel and rely on process-level isolation using namespaces and cgroups. They are lightweight and fast to deploy, but they do not offer the same degree of separation or flexibility when it comes to OS diversity or kernel-level control. Where speed and density are critical, containers are often preferred. But for applications that demand resource isolation, long-lived persistence, or cross-platform support, VMs remain the standard.
Why Virtual Machines Still Matter
Despite the growth of container technologies, virtual machines remain essential for many infrastructure use cases. They offer:
- Complete OS-level isolation for each workload
- Compatibility with legacy and non-Linux systems
- Consistent, image-based provisioning
- Stable environments for stateful applications
- Secure execution with hardware-enforced separation
From running staging environments to hosting production systems, VMs provide predictable behavior and long-term flexibility.
Conclusion
A virtual machine provides a complete, virtualized hardware environment that runs independently of the host system and other workloads. Backed by a hypervisor like KVM, VMs allow multiple operating systems to coexist securely on shared infrastructure while maintaining high performance and control.
Whether deployed in bare-metal environments or within hybrid cloud infrastructure, VMs remain foundational to modern compute, offering security, isolation, and full-stack compatibility without compromise.
