The PAV straps are PreVendome OEM straps from Panerai - there's a great writeup linked off the main page: http://www.paneristi.com/straps/PAV/pav_straps.html
These are highly sought after pieces that command an extremely high asking price. Collectors go nuts for them, and are some of the coolest straps around.
Dirks are straps from Dirk Grandry, a well known Paneristi, one who's forgotten more than most of us will ever know about Pre Vendome Panerai. Some more info, with pics, here: http://www.paneristi.com/straps/dirk/index.html
. They're only slightly less desirable than the PAV's.
the so-called "2nd Gen Dirks" are not Dirk straps at all. Major identifying characteristics are stitching that wraps the strap at the buckle end, and buckles where the "I" in PANERAI doesn't line up with opening of the buckle. These aren't as desirable as other straps...
Dirk is also doing another series of straps, and if you search the AC, you'll find links to his latest. Here's a courtesy link: http://www.network54.com/Forum/353393/message/1203939196/Dirk+Straps+-+New+Production
So, hopefully that clears the air a bit - there are also many undocumented prototype PAV's in the wild, but they are few and far between, and IMO< desirable as a novelty only. Most are some sort of red or green reptile skin...and that just doesn't do it for me personally.