I would like to know if it's allowed to run 10/3 Romex in 3/4 inch PVC pipe for 15 ft. outside under the floor of an open porch to be wired to a 30A power outlet for a portable 6500 watt generator.
No. Nonmetallic sheathed cable is not allowed in wet locations. NEC considers outdoor conduit a wet location.
You'll have to use THWN conductors, or other conductors/cables rated for wet locations.