  Discrete-variable (DV) and continuous-variable (CV) schemes constitute the
two major families of quantum key distribution (QKD) protocols. Unfortunately,
since the setup elements required by these schemes are quite different, making
a fair comparison of their potential performance in particular applications is
often troublesome, limiting the experimenters' capability to choose an optimal
solution. In this work we perform a general comparison of the major
entanglement-based DV and CV QKD protocols in terms of their resistance to the
channel noise, with the otherwise perfect setup, showing the definite
superiority of the DV family. We analytically derive fundamental bounds on the
tolerable channel noise and attenuation for entanglement-based CV QKD
protocols. We also investigate the influence of DV QKD setup imperfections on
the obtained results in order to determine benchmarks for the parameters of
realistic photon sources and detectors, allowing the realistic DV protocols to
outperform even the ideal CV QKD analogs. Our results indicate the realistic
advantage of DV entanglement-based schemes over their CV counterparts and
suggests the practical efforts for maximizing this advantage.
