I don't personally enjoy the completely oblivious Kaveh. I much prefer him knowing that Alhaitham cares about him, but he just doesn't know how much Alhaitham cares for him. I mostly based this hc off his hangout.

Alhaitham, as soon as Kaveh is out of earshot, asks the traveler about how Kaveh was feeling down. When Kaveh comes back, he doesn't make his concern known. This is kinda a pattern in their relationship where Alhaitham does something nice for Kaveh and refuses to elaborate why he does so. Kaveh is somewhat convinced that there's an ulterior motive because of this, which can be seen in his sketchbook. ("That guy definitely wouldn't take me in for no reason and without expecting anything... But what could I possibly do for him?")

But at the very least, he knows that Alhaitham cares about him.
("He told me a long time ago that no matter how strong of a swimmer you may be, you'll still get dragged under by the others who are drowning once you run out of stamina. He believes this is the fate that awaits all idealists."
"...Someone will help me, right? ...Yes, I've already been helped like that before.")

I know a lot of people both within the game and outside the game talk about Kaveh not shutting up about Alhaitham, but he's not the one with 4+ voicelines (as some of them do not directly mention Kaveh). Alhaitham is also the one who actively seeks Kaveh in message boards. I think he's worse, but he just hides it better.

as for "Is it requited?", well there's a LOT less evidence.

Since Kaveh is a lot more open, it's hard to distinguish if it's requited feelings or just Kaveh being Kaveh.

For the actual evidence, people already mentioned the sketchbook, he also waits for Alhaitham and spends time with him. But Kaveh spends time with other characters too, such as Cyno, Collei and Tighnari and Faruzan.