The current generation Toyota Previa (XR50) has been in production since 2006. It comes with a factory transponder immobiliser and keyless entry, but no alarm.
Two good Toyota Previa alarm options
For the models with a normal remote key, I highly recommend the Cobra AK4218 Toyota Upgrade alarm which works flawlessly with Toyota Remote.
For the higher spec model with the proximity smart key and push to start button the Cobra AK4615 CAN-BUS alarm is a great option.

Toyota Previa 2006-
The AK4218 perfectly with the Toyota remote key which makes it very user-friendly and effective.
A second press of the lock button on the remote disables the Ultrasonic Sensors.
The alarm comes with the Cobra 4310 siren as well as horn honk so is super loud should someone break in.
The AK4615 works with the smart key version
Both alarms share the same components. The following photos show how it looks in the vehicle:

Cobra Ultrasonic Sensors look tidy in the Previa

Cobra LED over-ride switch in the Previa
Easy Emergency Over-ride
The PIN code over-ride can be customized to any 4 digit number so the alarm can be disarmed if the remote battery fails. The following Video shows how it works: