anaasef,
As Amaury stated if your XLS didn't come with the Navigation system installed at the factory you'd have to replace it with an aftermarket unit.
Apparently the
Dual XDVDN8190 and the
Pioneer AVIC-N5 are the best fit in the Avalon according to Crutchfield.
To replace the original JBL system you'd probably need an aftermarket bracket such as the Scosche TA2086B ($19.95) or the Metra 99-8215.
You also may need to invest in some specific wiring harnesses to be able to use the stock amplifier and speakers as well as the steering wheel mounted controls.
I too am looking to upgrade my factory JBL system in my 2006 Avalon XLS.
00avxls provided some great information for us in a previous thread including pics, part numbers, and instructions.
Do a search for 00avxls and you'll find it.