There are many ways, many programs, and many processes that you could step through. Figure out what VM software you will use under Linux to pick your virtual disk file format, the sort when you are going to backup your existing install, and if anything is in your way like disk encryption.

If you can boot a live linux or pull the drive, look at qemu-img, I haven't used it myself in awhile for syntax but Google suggests a command like this will work for you

sudo qemu-img convert -f raw -O qcow2 -p /dev/sdX /path/to/disk.qcow2