I have installed ubuntu alongside windows 7/8 on my machine. Is it possible to run my windows instance as a KVM virtual machine?
Do I have to install windows again in KVM?
According to KVM's documentation Windows 7 and 8 are both supported, but may need extra drivers for the virtual disks, depending on what options you use (virtio is advantageous, but often requires drivers).
You will have to reinstall Windows again to run it in KVM.