The proliferation of cloud computing has compelled the research community to rethink fundamental design aspects of networked systems. However, the tools commonly used to evaluate new ideas have not kept abreast of the latest developments. Common simulation and emulation frameworks fail to provide scalability, fidelity, reproducibility and execute unmodified code, all at the same time. We present SELENA, a Xen-based network emulation framework that offers fully reproducible experiments via its automation interface and supports the use of unmodified guest operating systems. This allows out-of-the-box compatibility with common applications and OS components, such as network stacks and filesystems. In order to faithfully emulate faster and larger networks, SELENA adopts the technique of time dilation and transparently slows down the passage of time for guest operating systems. This technique effectively virtualizes the availability of host's hardware resources and allows the replication of scenarios with increased I/O and computational demands. Users can directly control the trade-off between fidelity and running-times via intuitive tuning knobs. We evaluate the ability of SELENA to faithfully replicate the behavior of real systems and compare it against existing popular experimentation platforms. Our results suggest that SELENA can ac-curately model networks with aggregate link speeds of 44 Gbps or more, while improving by four times the execution time in comparison to ns3 and exhibits near-linear scaling properties.