typedef struct _UNICODE_STRING {
USHORT Length;
USHORT MaximumLength;
PWSTR Buffer;
} UNICODE_STRING;
typedef UNICODE_STRING *PUNICODE_STRING;

typedef struct _STRING {
USHORT Length;
USHORT MaximumLength;
PCHAR Buffer;
} STRING;
typedef STRING *PSTRING;

typedef STRING ANSI_STRING;
typedef PSTRING PANSI_STRING;

To make life easier MS have extended kernel CRTL output() function with Z format specifier. This works for all kernel functions those understand formatted strings (e.g. sprintf, _vsnprintf, KdPrint/DbgPrint). For example:

PUNICODE_STRING pUStr;
PANSI_STRING pAStr;
…
KdPrint(("Unicode string: %wZ\n", pUStr));
KdPrint(("ANSI string: %Z\n", pAStr));

Though, you can use a little more complicated documented way. Btw, this form is suitable for printing byte array of strictly defined length.

KdPrint(("Unicode string: %*.*ws\n",pUStr->Length/sizeof(WCHAR),
pUStr->Length/sizeof(WCHAR), pUStr));
KdPrint(("Unicode string: %*.*S\n",pUStr->Length/sizeof(WCHAR),
pUStr->Length/sizeof(WCHAR), pUStr));
KdPrint(("ANSI string: %*.*s\n", pAStr->Length/sizeof(CHAR),
pAStr->Length/sizeof(CHAR), pAStr));

Or, if you want to take into account NULL-terminator, but limit output length to specified number of characters:

KdPrint(("Unicode string: %.*ws\n",
pUStr->Length/sizeof(WCHAR), pUStr));
KdPrint(("Unicode string: %.*S\n",
pUStr->Length/sizeof(WCHAR), pUStr));
KdPrint(("ANSI string: %.*s\n",
pAStr->Length/sizeof(CHAR), pAStr));