http://h34nug3iwqjf2376fmmzdhgqivckzemcujmarlptfqeqtq7vya47yyad.onion/2007/07/08/qemu-easy-and-fast-processor-emulator/index.html
While there are a few to choose from, qcow is the norm - it works well enough, and only takes up the space on the hard drive that it needs. If the virtual hard drive had a capacity of 5 gigabytes, but only contained 2 gigabytes of data, then it would only take up about 2 gigabytes on your real hard disk. So, now we want to get installing.