It depends on the security of the prison and their rules.
Some high security prisons don't even allow you to touch the prisoner and you must visit the prisoner through a glass wall and brick wall and use a phone to talk to them.
Some other prisons do a video visit if it's a high security prison so you won't be able to touch that person or kiss them unless you do so through the camera or glass.
However in most prisons that allow you to actually touch and see the prisoner in person they do allow a few seconds of hugging or kissing that prisoner.
You're not usually allowed to sit there and make love with the prisoner but a brief quick kiss of a few seconds is usually allowed.
The worse that would happen if kissing wasn't allowed and you did kiss that prisoner is that they would end the visit and tell you to leave.