At least we can all agree that Kiss and Queen are metal. 
It's difficult to define what differentiates heavy metal from hard rock exactly. Kiss deny being a heavy metal band ("Rock and Roll All Night" might indeed not be but what about the larger than life bombast of "Detroit Rock City"?) seeing themselves as a rock band and Queen were a band that played a variety of styles, but always with great panache (and they had a slightly split personality, their gigs were always at least 3/4 hard rock songs, the singles they put out did not mirror this). I guess this approaches heavy metal:
In the seventies, Deep Purple, Uriah Heep, Mountain, UFO, Nazareth, Alice Cooper, Thin Lizzy (Gorham/Robertson line up), Bad Company, Grand Funk Railroad, Led Zep, Rory Gallagher and Black Sabbath, even Hawkwind and Ten Years After, all were "heavy rock" to me and everyone I knew. With the exception of Black Sabbath all other bands staunchly refused the heavy metal tag though most of them had their metal moments. Even The Who's Live at Leeds has its metal moments. OTOH some bands claimed to be heavy metal like Blue Öyster Cult which more often than not weren't metal at all.
The term was only embraced by a wider group of bands at the end of the seventies with the advent of The New Wave of British Heavy Metal (which presupposed an old wave of British heavy metal, but no one wanted to be heavy metal back then?) even though that spawned bands such as Def Leppard which weren't heavy metal by a stretch. Priest, Saxon and Maiden though were proud to be heavy metal, Motörhead refused to be, which never made sense to me, they were just heavy metal with bad vocals. I know that Lemmy thinks that Ace of Spades is closer to Little Richard than to Paranoid of Black Sabbath but I disagree. And AC/DC believe to be sons of Chuck Berry, but the way their music is made to bludgeon you into submission is certainly a heavy metal'ish approach. American bands appropriated the name but what is more metal about, say, Quiet Riot's Metal Health "debut" (discounting the Sony stuff with Randy Rhoads they did earlier) than the Van Halen or Montrose debuts?
I never thought "heavy metal" was a very descriptive term and that the differences to hard rock and heavy rock are in the eye and ear of the beholder. My definition of all three to a martian would be "rock music where the distorted gutar is the dominant sound coloring, the music riff oriented and the greatest solo freedom is generally given to the guitar, all over a generally very steady rhythm in quarters, eights or sixteens". But of course there are metal songs that don't fit that mold and non-metal songs which do.
So are we left with what one judge once said on how he could differentiate erotic art from porn: "I know it when I see it."?
