Does it mean that the XLR output are not fully balanced....???
"Fully balanced" is a term that applies to amplifier construction and not the input or output.
Input can be balanced (in non fully balanced amp) by either Transformer or Instrumentation Amplifier. To make output balanced requires additional inverting stage (gain -1). XLR cables are less susceptible to noise, have locks and hidden (female) input pins. Other than greater noise immunity balanced cables should be less transparent than RCA cables because of this additional input and output stage.