As more and more virtual machines (VM) are packed into a physical machine, refactoring common kernel components shared by virtual machines running on the same physical machine could significantly reduce the overall resource consumption. The refactored kernel component typically runs on a special VM called a virtual appliance. Because of the semantics gap in Hardware Abstraction Layer (HAL)-based virtualization, a physical machine's virtual appliance requires the support of per-VM in-guest agents to perform VM-specific operations such as kernel data structure access and modification. To simplify deployment, these agents must be injected into guest virtual machines without requiring any manual installation. Moreover, it is essential to protect the integrity of in-guest agents at run time, especially when the underlying refactored kernel service is security-related. This paper describes the design, implementation and evaluation of a stealthy agent deployment and execution mechanism called SADE that requires zero installation effort and effectively hides the execution of agent code. To demonstrate the efficacy of SADE, we describe a kernel memory scanning virtual appliance that uses SADE to inject its in-guest agents, and report SADE's start-up overhead and run-time performance penalty on this virtual appliance.