If you are using virt-manager to mange your KVM instances, you can just open it, click on your instance, go to View → Details. There is a button "Add Hardware" were you can add Storage, which may also be a whole hard disk drive. Use 'VirtIO' as a means to access the hard drive - this works well with recent linux kernels, if your guest operating system is linux.
Beware that you do not use the hard disk on the hypervisor as long as the instance is running. E.g. if you mount a filesystem on the hypervisor that resides on the disk and is mounted from within the virtual machine, you are probably going to destroy the filesystem. So, a little extra caution pays well ;-)