DbgPrint and OutputDebugString display output to the host debugger Command window.
ULONG DbgPrint(PCH pchFormat, ...);
DbgPrint writes a string to the WinDbg KD Command window. The parameters of DbgPrint are the same as those used by the C runtime function printf. Only kernel-mode drivers can call DbgPrint. The ntddk.h file defines a KdPrint macro that is equivalent to DbgPrint, except that it is a no-op in free builds and outputs a string only in checked builds.
VOID OutputDebugString(LPCTSTR lpszOutputString);
OutputDebugString sends a string to the debugger of the calling process. When debugging a user-mode driver, OutputDebugString displays the string in the WinDbg Command window. OutputDebugString does not support the variable arguments of a printf formatted string.