What you're looking for is called booting from a VHD (Virtual Hard Disk). Assuming you have Windows 8 Pro, you can install it in a VHD file on your hard disk and boot from it as if it were installed on its own partition.
See this article for instructions (they're for the Developer Preview, but should apply to the RTM version as well).
If you have regular (non-Pro) Windows 8, you can try a program called VBoot, which should support any edition of Windows. They don't explicitly list Windows 8 support, though.