We explored the possibility of producing a two-dimensional electron gas (2DEG) in polar/polar (LaAlO₃)_m/(KNbO₃)_n perovskite superlattices that have N type and P type interfaces using the first-principles electronic structure calculations. Two different kinds of LaAlO₃/KNbO₃ superlattices were constructed, namely stoichiometric NP superlattice (NP-SL) and non-stoichiometric NN superlattice (NN-SL). We discovered that the NP-SL undergoes a transition from an insulating to a metallic state when LaAlO₃ has more than 3 unit cells. This reveals the completely spin-polarized two-dimensional hole gas (2DHG), as well as 2DEG with an interfacial charge carrier density of n ∼ 10¹³ cm⁻² and an electron effective mass of 0.240m_e (for 5 unit cells of LaAlO₃). In comparison, the NN-SL is intrinsically metallic, and when LaAlO₃ has 4.5 unit cells, the structure shows a 2DEG with a larger density (n ∼ 10¹⁴ cm⁻²) and a smaller electron effective mass (0.185m_e). In addition, the charge carrier properties are highly sensitive to the number of LaAlO₃ unit cells in the NP-SL model, while the size effect of LaAlO₃ is negligible for the NN-SL one. Our results demonstrate that electronic reconstruction at the interfaces of the stoichiometric structure can produce both the 2DHG and 2DEG, whereas extra electrons are introduced to form solely the 2DEG at the non-stoichiometric structure interfaces. This research provides fundamental insights into the different interfacial electronic properties and the primary mechanism responsible for the formation of polar/polar heterojunction LaAlO₃/KNbO₃ superlattices.