  Passive quantum key distribution (QKD) has been proposed for discrete
variable protocols to eliminate side channels in the source. Unfortunately, the
key rate of passive DV-QKD protocols suffers from sifting loss and additional
quantum errors. In this work, we propose the general framework of passive
continuous variable quantum key distribution. Rather surprisingly, we find that
the passive source is a perfect candidate for the discrete-modulated continuous
variable quantum key distribution (DMCV QKD) protocol. With the phase space
remapping scheme, we show that passive DMCV QKD offers the same key rate as its
active counterpart. Considering the important advantage of removing side
channels that have plagued the active ones, passive DMCV QKD is a promising
alternative. In addition, our protocol makes the system much simpler by
allowing modulator-free quantum key distribution. Finally, we experimentally
characterize the passive DMCV QKD source, thus showing its practicality.
