Start Windows in virtual machine from real hard drive partion

VMWare products can map physical disks to virtual disks, both on Linux and Windows:

http://www.vmware.com/support/ws55/doc/glossary_ws.html#wp1018930

Physical disk

— A hard disk in a virtual machine that is mapped to a physical disk drive or a partition of a drive on the host machine. A physical disk is also referred to as a raw disk. A virtual machine's disk can be stored as a file on the host file system (see Virtual disk) or on a local hard disk. When a virtual machine is configured to use a physical disk, VMware Workstation directly accesses the local disk or partition as a physical device (not as a file on a file system). It is possible to boot a previously installed operating system on an existing partition within a virtual machine environment. The only limitation is that the existing partition must reside on a local IDE or SCSI drive.

In xen you can use the device and give it directly to your guest. You can use lvm:

disk        = [
                  'phy:/dev/volg1/thedisk,sda1,w',
                  'phy:/dev/volg1/theswap,sda2,w',
              ]

or directly a harddrve partition:

disk        = [
                  'phy:/dev/sda1,sda1,w',
              ]

you also can give a complete Disk to a guest, and let them partiton it:

disk        = [
                  'phy:/dev/sdb,sdb,w',
              ]

Easier: sudo qemu /dev/sdX will launch a virtual machine from an actual disk ;) qemu is free software, usually present in your distro's repositories