1. A system for providing a streaming-based portable application under selective conditions, the system comprising: a streaming server configured to store an application execution code provided inside of the system necessary to execute a portable application; and a client configured to provide a virtualization of the application execution code, streaming the application execution code from the streaming server through a network, and managing streamed application images, the client comprising: a memory; a portable application kernel engine configured to provide the virtualization of the application execution code necessary to execute the portable application, and executing a portable application service; an application manager configured to manage the streamed application images; an application streaming manager configured to stream the application execution code from the streaming server according to a request of the portable application kernel engine or the application manager; a cache configured to store the application execution code received from the streaming server; and an operating system in which the portable application kernel engine is embedded, wherein the application manager manages the portable application such that an initial execution of the portable application is differently performed according to conditions, the conditions including each of a case where the portable application is completely stored in the cache, a case where the portable application is not completely stored, a case where a user wants to stream the execution code of the portable application at a time, and a case where the user wants to stream the execution code of the portable application whenever necessary, and wherein the portable application kernel engine reads the application execution code from the cache when an execution application type is the portable application, and continues to execute the portable application, notifies the application streaming manager of a non-existence of the cache of the corresponding execution code when the execution application type is for application streaming of the portable application and when the corresponding execution page does not exist in the cache, requests the streaming server to transmit the corresponding application execution code through the network, and receives the corresponding execution page, and stores the entire application execution code in the cache, sets the corresponding application execution code from the application streaming to the portable application, notifies the application streaming manager of the reception result of the application execution code, and continues to execute the application.