The EU-funded XtreemOS project implements a grid opera\-ting system (OS) transparently exploiting distributed resources through the SAGA and POSIX interfaces. XtreemOS uses an integrated grid checkpointing service (XtreemGCP) for implementing migration and fault tolerance for grid applications. Checkpointing and restarting applications in a grid requires saving and restoring distributed/parallel applications in distributed heterogeneous environments. In this paper we present the architecture of the XtreemGCP service integrating existing system-specific checkpointer solutions. We propose to bridge the gap between grid semantics and system-specific checkpointers by introducing a common kernel checkpointer API that allows using different checkpointers in a uniform way. Our architecture is open to support different checkpointing strategies that can be adapted according to evolving failure situations or changing application requirements. Finally, we discuss measurements numbers showing that the XtreemGGP architecture introduces only minimal overhead.