1. An apparatus for virtual desktop service, comprising: a connection manager configured to perform an assignment task of assigning a virtual machine to a user terminal using the virtual desktop service; a resource pool configured to provide software resources including an operating system, applications, and user profiles to the virtual desktop service; and a virtual machine infrastructure configured to support hardware resources, wherein the connection manager further performs a coordination task of coordinating a delivery protocol used between the user terminal and servers that provide the virtual desktop service, wherein the virtual machine infrastructure supports high availability (HA) with predefined configuration parameters, wherein the connection manager and the virtual machine infrastructure including a first server and a second server are clustered for the high availability (HA), wherein a virtual machine is relocated to the activated second server when a failure occurs on the first server to which the virtual machine is currently allocated, wherein a delivery function for the delivery protocol determines factors for the virtual desktop service based on user state information, and wherein the factors include screen resolution, compression rate of video and audio, and kind of video and audio codec.