The reason DVDV patch is needed is to fool the mechacon into authenticating the disc.
Letting the ps2 access the disc that is inserted is handled by a dedicated microcontroller known as the "mechacon" (mechanics controller) If the mechacon fails to validate the disc, then the IOP CANNOT access it, because the drive is locked.
The following types of media are valid.
1)original nonimport ps1 discs
2)original non import Ps2 cd and dvd
3) audio CD
4) DVD Video
a "DVDV" appears to the ps2 to be a dvd video disc, so the mechacon unlocks the drive.
Modchips also fool the mechacon.