Answers.com defines a historical novel thus:
"the action takes place during a specific historical period well before the time of writing (often one or two generations before, sometimes several centuries), and in which some attempt is made to depict accurately the customs and mentality of the period. The central character—real or imagined—is usually subject to divided loyalties within a larger historic conflict of which readers know the outcome. The pioneers of this genre were Walter Scott and James Fenimore Cooper; Scott's historical novels, starting with Waverley (1814), set the pattern for hundreds of others: outstanding 19th‐century examples include Victor Hugo's Notre Dame de Paris (1831), Dumas père's Les Trois Mousquetaires (1844), Flaubert's Salammbô (1862), and Tolstoy's War and Peace (1863–9). While the historical novel attempts a serious study of the relationship between personal fortunes and social conflicts, the popular form known as the historical or ‘costume’ romance tends to employ the period setting only as a decorative background to the leading characters. "
Well, that sort of says it all. I can imagine the bristles rising all over the globe as romance writers and readers read the last sentence, but remember, girls, I didn't write it - I'm only quoting it!
I think there is a bridge between the two genres inhabited by less serious historical novels and more serious romance historicals. How deep a reader feels the bridge extends in either territory depends on personal taste. One woman might refuse to step on the bridge, and another might head a mile into the other territory. It is good that the bridge is there, because it means everyone can find the right level for them.
Is the black woolly creature in the pic alpaca, lama or vicuna? I can never remember which is which, but I hope this little creature (s/he was very young when I took his/her picture up near Harbottle in the autumn) is tucked away in a nice warm barn somewhere.