Let's consult allmusic, which is (generally speaking) good for these things. in their definition/history of heavy metal, they offer these excerpts (the full text is in the link):

"Of all rock & roll's myriad forms, heavy metal is the most extreme in terms of volume, machismo, and theatricality. There are numerous stylistic variations on heavy metal's core sound, but they're all tied together by a reliance on loud, distorted guitars (usually playing repeated riffs) and simple, pounding rhythms...

"The first seeds of heavy metal were sown in the British blues movement of the '60s, specifically among bands who found it hard to adjust to the natural swing of American blues. The rhythms became more squared-off, and the amplified electric instruments became more important, especially with the innovations of artists like the Kinks, the Who, Jimi Hendrix, Cream, and the Jeff Beck Group. Arguably the first true metal band, however, was Led Zeppelin. Initially, Zep played blues tunes heavier and louder than anyone ever had, and soon created an epic, textured brand of heavy rock that drew from many musical sources. Less subtle but perhaps even more influential was Black Sabbath, whose murky, leaden guitar riffs created a doomy fantasy world obsessed with drugs, death, and the occult."

Their definition of hard rock is only a little more concrete:

"Hard rock is a term that's frequently applied to any sort of loud, aggressive guitar rock, but for these purposes, the definition is more specific. To be sure, hard rock is loud, aggressive guitar rock, but it isn't as heavy as heavy metal, and it's only very rarely influenced by punk (though it helped inspire punk). Hard rock generally prizes big, stadium-ready guitar riffs, anthemic choruses, and stomping, swaggering backbeats; its goals are usually (though not universally) commercial, and it's nearly always saturated with machismo. With some bands, it can be difficult to tell where the dividing line between hard rock and heavy metal falls, but the basic distinction is that ever since Black Sabbath, metal tends to be darker and more menacing, while hard rock (for the most part) has remained exuberant, chest-thumping party music. Additionally, while metal riffs often function as stand-alone melodies, hard rock riffs tend to outline chord progressions in their hooks, making for looser, more elastic jams should the band decide to stretch out instrumentally."

I think the dividing line is Satan.