Abstract: Systematic comparative studies on the effects of three typical cavitation induction methods—ultrasound, spark discharge, and laser ablation—on bubble dynamics, energy density, and energy conversion efficiency remain limited. In this study, high-speed imaging and Gilmore–Nasg numerical simulations were employed to investigate the dynamic behaviors and energy conversion characteristics of cavitation bubbles generated by these three methods in deionized water. The results show that, although spark- and laser-induced cavitation involve higher input energies, ultrasonic cavitation exhibits strongly nonlinear dynamic evolution. This is attributed to the initially rarefied state of the gas inside the bubble and the synergistic compressive effect of the positive-pressure phase of the acoustic field during bubble collapse. The energy conversion efficiency of ultrasonic cavitation reaches 2.41%, which is significantly higher than that of spark-induced cavitation (0.10%) and laser-induced cavitation (0.03%). Moreover, the collapse velocity of ultrasonic cavitation bubbles is 2.74 and 4.06 times greater than that of spark- and laser-induced cavitation bubbles, respectively. By fitting the temporal evolution curves of the bubble radius, an inversion method for determining the polytropic exponent of the gas inside the bubble was established. Based on this method, the thermodynamic state characteristics of the gas within the bubble were further analyzed, providing both experimental and theoretical support for elucidating cavitation mechanisms and regulating local energy density.