You might wonder—why not use VirtualBox VDI or VMWare VMDK? The answer is performance and flexibility. QCOW2 is native to KVM, which offers near-bare-metal performance on Linux hosts. For a resource-conscious OS like Windows 8 (which runs comfortably on 2GB of RAM and 20GB of storage), QCOW2’s thin provisioning is a match made in heaven.
Most users prefer a GUI. In tools like virt-manager (the standard Linux KVM manager), you select "Create a new virtual machine," choose your Windows 8 ISO, and on the storage step, select "Select or create custom storage" and ensure the format is set to QCOW2.
qemu-img convert -f raw -O qcow2 win8.raw win8.qcow2