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The presence of ferroelectricity in HfO2-based thin films have revitalized the interests in using ferroelectrics at the nanoscale. To understand the origin of ferroelectricity in this silicon-compatible ferroelectric, we investigated the kinetic effects of phase transitions in HfO2 thin films by quantifying the transition barriers between different polymorphs of hafnia with density-functional-theory calculations and variable-cell nudged elastic band technique. We found that the transition from the tetragonal phase to the polar orthorhombic phase is a fast process kinetically under clamping. We further explored the pryoelectricity in HfO2 with both first-principles lattice dynamics and ab initio molecular dynamics calculations. Unlike most conventional pyroelectrics, the large pyroelectricity of HfO2 results from the secondary effect which is intimately related to the negative longitudinal piezoelectric effect.