PWM and MPPT are actually two different things, but marketing confuses the consumer and can lead to urban myths. Warning - long, but perhaps entertaining!
A mythical story might help explain:
You are stranded on an island with a nominal 12v panel that actually puts out 20v unregulated. You have a nominal 12v battery that is rated with a CV charge of no greater than 14.4v.
Long ago, the controller broke. All you have left is an on/off toggle switch as your only means to prevent overcharge.
Early years of use: You wire it up, and sitting at the toggle switch, you watch your voltmeter on the battery terminals rise to 14.4v. You open the switch. You let the battery "settle" back to say 13.2v and then close the switch again. Watching for 14.4v, you open the switch and repeat this process. As the battery charges, the time interval between you opening and closing the switch gets longer and longer. Unfortunately, this results in a battery that is not fully charged before the sun sets.
This is also known as "hysteresis" charging.
In a desire to be more efficient, you come across a Morningstar pwm controller document in a cave. AHA! You read that the pwm frequency is 38 khz. That simply means that instead of doing an inefficient hysteresis, you just need to flip the switch on and off 38.8 thousand times a second with your hands.
This means that the actual panel voltage of 20v is applied to the battery in very very short connections, thirty-eight-thousand times per second. But by doing this, the NET EFFECT is that of applying a 14.4v CV when time spent at connection is considered.
Amped on coffee, stashed sports-drink, and the island's own go-go juice, you sit at the table and flick the on-off toggle switch 38000 times a second. Villagers witness a veritable buzz of activity at your table. They can't even see your hand moving!
The villagers are happy since charging the battery in this way is a vast improvement over the early years of hysteresis. Being promoted to village elder, they are expecting even more performance than this! How can this possibly get even better?
Ah, knowing that in the end, all you have to do is prevent the battery from exceeding 14.4v from a *NET OVERALL* high-speed toggle switch, what if we allow that speed to vary and by careful use of your ammeter skills, you manually raise or lower the speed so that you always get the highest current going into the battery as possible?
Like tracking your prey on the island, you adjust your pwm hand-toggling speed according to need and feedback to make it the best possible all the time. Unlike the early method, where given enough go-go juice to toggle that switch exactly at 38000 times a second, AND read a book in the other hand, now you need to concentrate a little more to adjust that speed as conditions change.
The villagers have now promoted you to being the "Maximum Power Point Tracker".
Sorry for the editorial license. The overall point to the story is that MPPT controllers use varying pwm techniques for best benefit. PWM controllers are just using this method of nomenclature to differentiate themselves early on from the older "hysteresis" controllers. MPPT is an *additional* measure added to a controller using PWM.
Ie, your panel is never actually "seeing" the full unregulated voltage from the panel because the full-panel-voltage switching is so fast, that the *net effect* is that of a lower voltage overall.