The brand of "Heavy Metal" that existed in the early-to-mid 70's might be primitive compared to the "Metal" genres of the 80's (Hair, Speed, Thrash), but Black Sabbath was considered Heavy Metal in the 70's (as was Led Zeppelin and Deep Purple), and just because Metal's sound has evolved since the early 70's, there's no reason to reclassify them as "Hard Rock" 40 years later.
Not everything that Zeppelin, Sabbath and Purple ever recorded was Heavy Metal, but there were certain songs (e.g.: Smoke on the Water, Highway Star, Paranoid, Sweet Leaf, Whole Lotta Love and Immigrant Song) that went beyond the parameters of what Hard Rock was at the time. That's the difference.
Though Jimi and Cream had their moments in the 60's with songs like Purple Haze and Sunshine of Your Love, which is why I mentioned those two songs as perhaps being the first Heavy Metal song in another thread.