  The kagome superconductor CsV$_3$Sb$_5$ hosts a variety of charge density
wave (CDW) phases, which play a fundamental role in the formation of other
exotic electronic instabilities. However, identifying the precise structure of
these CDW phases and their intricate relationships remain the subject of
intense debate, due to the lack of static probes that can distinguish the CDW
phases with identical spatial periodicity. Here, we unveil the competition
between two coexisting $2\times2\times2$ CDWs in CsV$_3$Sb$_5$ harnessing
time-resolved X-ray diffraction. By analyzing the light-induced changes in the
intensity of CDW superlattice peaks, we demonstrate the presence of both
phases, each displaying a significantly different amount of melting upon
excitation. The anomalous light-induced sharpening of peak width further shows
that the phase that is more resistant to photo-excitation exhibits an increase
in domain size at the expense of the other, thereby showcasing a hallmark of
phase competition. Our results not only shed light on the interplay between the
multiple CDW phases in CsV$_3$Sb$_5$, but also establish a non-equilibrium
framework for comprehending complex phase relationships that are challenging to
disentangle using static techniques.
