In the navigation pane (far left column) in Hyper-V Manager, select your host then right click it and choose ->New->Virtual Machine and go through the wizards and create your VM (hard disk, memory, number of CPUs, etc., and then install your XP operating system.
There is not really a true XP Mode type environment available in Windows 8, so you will need media as well as a proper C.O.A. (product key) as they are no longer provided for free.