As you know, Thomas Hardy considered Tess in Tess of the D'Urbervilles a pure woman since the subtitle of the work is "A Pure Woman." I have not taught or researched the novel, so you may get a far more detailed answer from another editor, but I can give you the obvious meaning of Hardy's subtitle.
Tess suffers throughout her life for, in the eyes of society, being impure. She is used and abused by society in general and men in particular. With his subtitle, along with the text of the novel, of course, Hardy is suggesting that there is far more to being pure, to being a decent human being, to being a loving person, than just whether or not someone has had sex out of wedlock. Hardy is commenting on prudish, hypocritical Victorian values. Tess is pure, Hardy seems to be suggesting, because of her personality and her character, etc., regardless of what society thinks of her.
She is certainly a better human being than any of the men who abuse her and judge her.
There is, therefore, of course, as you've probably concluded, an element of irony in Hardy's subtitle.