Take a look at the definitions in the Cambridge online dictionary. The definitions are very similar but the examples start to show the difference:
Ardour: Definition-
great enthusiasm or love:
Example-
His ardour for her cooled after only a few weeks.
Fervour: Definition-
Strong and sincere beliefs
Examples-
The country was swept by patriotic fervour.
nationalist/religious fervour
The difference is that "Ardour" is a personal emotion, pure and simple. It is the result of being swept up by an emotional response to a stimulus, very often a sexual one, while "Fervour" is an emotion resulting from a more intellectual conviction or a social pressure.
One feels ardour for someone to whom one is sexually attracted or for works of art, literature, music and so on and, although one might extol the producer to friends and acquaintances ("I think Hockney's marvelous"marvellous", "Isn't Ed Sheeran wonderful?") there is no consistent attempt to convert them to one's own way of thinking.
Fervour, on the other hand, almost always relates to a political or religious conviction or a commitment to a community, state or people. For instance, one might be driven by nationalist fervour, particularly in times of war or crisis, or one might be fervent in one's commitment to protecting the environment. People with fervour for a subject usually try to convert other people to their way of thinking.
It is possible to be driven by both ardour for a particular object of affection and by fervour to protect it. For example, many environmental campaigners have been driven by an emotional response amounting to ardour for a particular landscape, animal or plant to wage a fervent campaign to protect it.
Primatologist and conservationist Dian Fossey is a good example of this. Her commitment to mountain gorillas was almost certainly driven by the ardour of her affection for them but the fervour of her campaign to protect them led both to a greater degree of protection for them and, probably, to her murder.
