  We employ polarization-resolved electronic Raman spectroscopy and density
functional theory to study the primary and secondary order parameters, as well
as their interplay, in the charge density wave (CDW) state of the kagome metal
AV3Sb5. Previous x-ray diffraction data at 15K established that the CDW order
in CsV3Sb5 comprises of a 2x2x4 structure: one layer of inverse-star-of-David
and three consecutive layers of star-of-David pattern. We analyze the lattice
distortions based the 2x2x4 structure at 15K, and find that U lattice
distortion is the primary order parameter while M and L distortions are
secondary order parameters for Vanadium displacements. This conclusion is
confirmed by the calculation of bare susceptibility that shows a broad peak at
around qz=0.25 along the hexagonal Brillouin zone face central line (U-line).
We also identify several phonon modes emerging in the CDW state, which are
lattice vibration modes related to V and Sb atoms as well as alkali atoms. The
detailed temperature evolution of these modes' frequencies, HWHM, and
integrated intensities support a phase diagram with two successive structural
phase transitions in CsV3Sb5: the first one with a primary order parameter
appearing at TS=94K and the second isostructural one appearing at around 70K.
