# QEMU KVM Acceleration

**KVM (Kernel-based Virtual Machine)** accelerates QEMU when the guest and host architectures match. QEMU hands off instruction execution to the CPU's hardware virtualization extensions, yielding near-native VM performance instead of the much slower software instruction translation (TCG).

```bash
qemu-system-x86_64 -enable-kvm -m 4G -smp 4 -hda disk.img    # with KVM
qemu-system-x86_64 -m 4G -smp 4 -hda disk.img                # without (TCG, slow)
```

KVM requires the host CPU to support virtualization extensions: Intel's `vmx` or AMD's `svm`. Check with:

```bash
grep -c '(vmx|svm)' /proc/cpuinfo      # nonzero means supported
kvm-ok                                  # Ubuntu utility
```

KVM also requires the `kvm` kernel module to be loaded and the host to have `/dev/kvm` readable by the QEMU process. Check with:

```bash
ls -l /dev/kvm              # should be readable by your user/group
lsmod | grep kvm            # should show kvm and kvm_intel or kvm_amd
```

When `-enable-kvm` is used and KVM is unavailable, QEMU falls back to TCG (Tiny Code Generator)—its software CPU emulator, which is much slower. TCG is what enables cross-architecture emulation (ARM on x86) at all, but it trades speed for flexibility.

KVM cannot emulate different architectures—only the same one as the host. Run `qemu-system-aarch64` on an ARM host to get KVM; on x86, only `qemu-system-x86_64` can use `-enable-kvm`.

For x86 VMs, use `-enable-kvm` unless testing legacy CPUs. For embedded (ARM, RISC-V) testing, KVM is unavailable; QEMU falls back to TCG (slower but still useful).