The exact method depends on the particular VM implementation you're using and details of the hardware and VM configuration. The basic idea is that you simply point the VM configuration at the disk partition where Ubuntu lives.
If Ubuntu was installed into a single partition in a generic PC, that may be all you need to do.
If you used multiple partitions, you may have to boot into some kind of safe or rescue mode and modify
/etc/fstab. This would be the case, for instance, if Ubuntu booted natively sees the hard disk as
/dev/sda while it appears as
/dev/hda under the VM.
Another trap you could run into is configuring the VM to use a disk controller that isn't in the Ubuntu kernel or initrd.
Basically, you've opened a can of worms here. Maybe there are no worms in the can, maybe lots. You won't know how many until you look.