This work is devoted to the modeling of space charge dominated particle beams in the paraxial approximation with several types of external focusing fields (uniform, periodic and alternating gradient). A solid mathematical background for numerical beam simulation is established. The Kapchinsky-Vladimirsky distribution which can be matched exactly or numerically to the focusing channel is first studied. Then the matched KV beam is used to approximately match arbitrary beams. Moreover Water-Bag and Maxwell Boltzmann beams are studied to give analytical solution for code validation. Finally, numerical simulations are presented in different configurations.