  We report $^{121}$Sb nuclear quadrupole resonance (NQR) measurements on
kagome superconductor CsV$_3$Sb$_5$ with $T_{\rm c}=2.5$ K. $^{121}$Sb NQR
spectra split after a charge density wave (CDW) transition at $94$ K, which
demonstrates a commensurate CDW state. The coexistence of the high temperature
phase and the CDW phase between $91$ K and $94$ K manifests that it is a first
order phase transition. The CDW order exhibits Tri-Hexagonal deformation with a
lateral shift between the adjacent kagome layers, which is consistent with $2
\times 2 \times 2$ superlattice modulation. The superconducting state coexists
with CDW order and shows a conventional s-wave behavior in the bulk state.
