I think there's a little mistake in tha story of TLK2. I'll do my best to explain:

When Kovu is first accepted by Simba in the pridelands, because he had saved Kiara, it was all part of Zira's plan. In fact, the next morning, he truely tries to kill Simba. When he learns what 'fun' means and he falls in love with Kiara, his mind changes completelly, but he still feels guilty for his evil past, and he is really afraid of telling Kiara the truth.

After the outsider's ambush, Simba tells everyone about Kovu being part of Zira's plot. Kiara doesn't believe a word, thinking Kovu is completely inocent; and well, Kovu was not completely inocent. OK, he had nothing to do with the ambush, but he had been part of Zira's plot. After that, in the 'Love will Find a Way' scene, Kovu looks very sad at first, because he thinks Kiara would never forgive him, but then he reopens his eyes, like thinking: “What is she doing here, so far from the pridelands? If she's here... she must be... looking for me! SHE HAS FORGIVEN ME!”.

But the truth is, that Kiara didn't really konw that Kovu had once been part of Zira's plan.

In conclusion: at the end, Kovu thinks Kiara still loves him, even after being told about his past, but Kiara hadn't believe what Simba told her, and she believed that Kovu had really left the outsiders from the beggining.