They have no diplomatic relations, they have no official channels. However, a situation can happen any time as Trump wants to send a message to Kim Jong Un, or vice versa. Closing all communication channels doesn't seem very wise to me. I think it is unlikely that any side would want to do it. For example, if Kim Jong Un wants to say:
In a half hour, we will send another missile into Hawaii, but don't worry, it won't reach it.
He has to be able to send this message, directly or indirectly, to Trump, because Trump will decide what to do if the military signals that there is a new missile launch. Neither of them wants a nuclear war.
Consider if they make another U.S. prisoner. Some channel must exist where they negotiate the price of his/her release. How does it work?