QKD aims at transmitting by quantum means random bits that are shared between sender and receiver, and then can serve as a shared key for a protocol over classical channels, like the OTP + Carter-Wegman MAC, or (more often in practice for performance reasons) an authenticated block cipher.
QSDC aims at directly transmitting by quantum means data bits chosen by the sender. As a foundational article defines it:
As a secure direct communication, it must satisfy two requirements. First, the secret messages should be read out directly by the legitimate user Bob when he receives the quantum states, and no additional classical information is needed after the transmission of qubits. Second,
the secret messages which have been encoded already in the
quantum states should not leak even though an eavesdropper
may get hold of the channel.
In both QKD and QSDC, in order to resist a man-in-the-middle attack, it is necessary to have another independent mean of trusted communication. It's used to send at least one message trusted to get thru to the correct destination without modification; plus this message must be transmitted with confidentiality if that trusted communication is to occur before the quantum communication. Depending on stakes, that trusted message can be sent by SMS, Fedex carrying a temper-evident opaque envelope, or a diplomat carrying a briefcase.
