Phoronix: QED: A New, High Performance QEMU Disk Format
Linux-KVM mentions QED, the new QEMU Enhanced Disk format. This new disk format for QEMU/KVM is designed to be much faster than QCOW2 and other existing disk formats available to virtualization users...
trim support doesn't necessarily require non-raw file system.
Actually, I'm far from being an expert in this field, but I would imagine that implementing virtualized hardware w/ trim support is somewhat easier on raw-FS-partition, as qemu can simply pass the trim request (w/ address translation) from the guest to the underlying hardware.
... but as I as I said, I'm guessing
As for performance, I would guess that raw will have the upper hand given the same optimization on the qemu side (E.g. write combination, etc). But given the fact that most of the effort is spent on optimizing advanced image types (qcow, etc), it's entirely possible that raw will not win the benchmark(s).