Alkali-metal incorporation is widely employed to improve the performance and stability of organic halide perovskites, yet the underlying doping mechanisms and diffusion pathways remain poorly understood. Here, we use ultraviolet photoemission spectroscopy to probe the evolution of the electronic structure of CH3NH3PbX3 (X = I, Br, and Cl) single crystals upon potassium and cesium deposition. We observe two distinct regimes governed by halide chemistry: facile bulk diffusion in CH3NH3PbI3 and CH3NH3PbBr3, in contrast to surface-confined accumulation in CH3NH3PbCl3, arising from kinetically hindered alkali migration in the chloride perovskite. Temperature-dependent measurements of the doped crystals further detect opposite valence band shifts for CH3NH3PbBr3 relative to CH3NH3PbI3 and CH3NH3PbCl3, highlighting the role of surface termination and charge redistribution. Notably, alkali metal deposition induces substantial energy level shifts while leaving band dispersions largely unchanged, indicating a predominantly interfacial doping mechanism. These results establish a direct link among halide composition, diffusion behavior, and electronic structure evolution, providing microscopic insight into alkali-metal interactions in perovskites.