KVM vs LXC: Choosing the Right Virtualization
Proxmox VE offers two distinct virtualization technologies: KVM for full hardware virtualization and LXC for lightweight containers. Understanding their differences helps you choose the right tool for each workload.
Side-by-Side Comparison
KVM Virtual Machines
What It Is
- Full hardware virtualization
- Complete operating system emulation
- Runs its own kernel
- Hardware-assisted (Intel VT-x/AMD-V)
Best For
- Running Windows or non-Linux OSes
- Different kernel requirements
- Maximum isolation needed
- Testing kernel modifications
- Legacy applications
Advantages
- Any OS: Windows, BSD, even unusual OSes
- Strong isolation: Complete separation from host
- Custom kernel: Each VM has own kernel
- Legacy support: Old OSes and applications
- Snapshots: Full VM state snapshots
Limitations
- Higher resource overhead
- Slower boot times (30-60 seconds)
- More memory usage per instance
- Lower density per host
Performance
- CPU: ~90-95% native with KVM
- Memory: Overhead for full OS
- I/O: Good with virtio drivers
- Network: Near-native with virtio-net
LXC Containers
What It Is
- OS-level virtualization
- Shares host kernel
- Process and filesystem isolation
- Linux namespaces + cgroups
Best For
- Linux-only workloads
- High-density deployments
- Development environments
- Microservices
- Web servers and applications
Advantages
- Lightweight: Minimal overhead
- Fast: Boot in 1-2 seconds
- Efficient: 10-100x more containers per host
- Simple: Easier to manage and backup
- Performance: Near-native performance
Limitations
- Linux only (no Windows)
- Shares host kernel version
- Less isolation than VMs
- Cannot modify kernel
Performance
- CPU: ~99% native performance
- Memory: Minimal overhead
- I/O: Native speed
- Network: Native performance
Technical Comparison
| Aspect | KVM (Virtual Machines) | LXC (Containers) |
|---|---|---|
| Kernel | Guest has own kernel | Shares host kernel |
| Boot Time | 30-60 seconds | 1-3 seconds |
| Memory Overhead | ~100-500 MB per VM | ~1-10 MB per container |
| Density | 10-50 VMs per host | 100-1000 containers per host |
| Isolation | Complete (hardware-level) | Process-level |
| OS Support | Any x86/x64 OS | Linux distributions only |
| Snapshots | Full VM state + disk | Filesystem only |
| Live Migration | Yes (seconds of pause) | Offline migration only |
| GPU Passthrough | Yes (PCIe passthrough) | Limited (no direct hardware) |
| Security | Hardware isolation | Namespace isolation + AppArmor |
Use Case Decision Matrix
Choose KVM When:
- Running Windows, BSD, or other non-Linux operating systems
- Need to test different kernel versions or modules
- Require maximum isolation for security or compliance
- Running untrusted code that needs complete separation
- Need GPU passthrough for graphics workloads
- Running applications with specific kernel requirements
- Need full VM state snapshots for testing
- Migrating from VMware or other VM platforms
Choose LXC When:
- Running Linux applications and services
- Need high density (many instances per host)
- Want fast boot times and minimal overhead
- Deploying microservices architecture
- Building development and testing environments
- Running web servers, databases, application servers
- Need to maximize resource utilization
- Cost-conscious deployments (more workloads per hardware)
Real-World Examples
KVM Use Cases
Windows Servers
Active Directory, Exchange, SQL Server, or any Windows application.
Development Workstations
Full desktop environments for remote workers (VDI).
Testing Different Kernels
Kernel development, driver testing, or OS research.
Legacy Systems
Old operating systems or applications requiring specific environments.
LXC Use Cases
Web Servers
NGINX, Apache, Node.js applications with fast deployment.
Database Servers
MySQL, PostgreSQL, MongoDB with near-native performance.
CI/CD Agents
Build agents, test runners that need fast creation/destruction.
Microservices
Individual service components in a distributed application.
Can You Mix Both?
Yes! Use Both on the Same Host
Proxmox VE allows you to run KVM VMs and LXC containers on the same physical host, managed through the same interface. This flexibility lets you optimize each workload individually.
Common Pattern:
- Windows servers and databases run in KVM VMs
- Linux web servers and applications run in LXC containers
- Development environments use LXC for speed
- Production critical services use KVM for isolation
Performance Benchmarks
| Test | Native | KVM | LXC |
|---|---|---|---|
| CPU (Integer) | 100% | ~95% | ~99% |
| Memory Bandwidth | 100% | ~90% | ~99% |
| Disk I/O (Sequential) | 100% | ~85-95% (virtio) | ~99% |
| Network Throughput | 100% | ~90-95% (virtio-net) | ~99% |
| Startup Time | N/A | 30-60 seconds | 1-3 seconds |
Note: Actual performance varies based on configuration, workload, and hardware.
Security Considerations
KVM Security
- Hardware-enforced isolation via CPU virtualization extensions
- Each VM has completely separate kernel and memory space
- Compromise of one VM doesn't affect others or host
- Suitable for multi-tenant environments
- Can run untrusted code safely
LXC Security
- Process isolation via Linux namespaces
- AppArmor/SELinux mandatory access control
- Seccomp syscall filtering
- Unprivileged containers (user namespace mapping)
- Kernel exploits could affect host
- Best for trusted workloads or single-tenant
Migration Between KVM and LXC
- VM to Container: Install application in container, migrate data, test thoroughly
- Container to VM: Create VM with same OS, install application, migrate data
Both KVM and LXC have their place in a modern infrastructure. By understanding their strengths and limitations, you can make informed decisions that optimize performance, density, and cost for your specific needs.