I have an IOGear dual DVI KVM switch that I use with HDMI via converter dongles.
All of the high end IOGear KVMs work by grabbing the monitor EDID when either the KVM or monitor is turned on, or if the monitor is unplugged and switched. This is then stored and passed on the to computers, so they think they are always connected to the monitor and can stay at the correct resolution. Based on descriptions, this appears to be "emulated" EDID, and it works flawlessly and is what you should be looking for.
Otherwise, it's just like manually unplugging and plugging the monitors, which means 5-30 seconds to sync up, plus desktop resolution changes and windows get moved around, etc.