Differences between ATmega328/P and ATmega328PB (PDF!) application note from Atmel:
This application note assists the users of Atmel® ATmega328 variants to understand the differences and use Atmel ATmega328PB.
ATmega328PB is not a drop-in replacement for ATmega328 variants, but a new device. However, the functions are backward compatible with the existing ATmega328 functions. Existing code for these devices will work in the new devices without changing existing configuration or enabling new functions. The code that is available for your existing ATmega328 variants will continue to work on the new ATmega328PB device.
The ATmega328PB is the first 8-bit Atmel AVR® device to feature the successful Atmel QTouch® Peripheral Touch Controller (PTC).
For differences in errata, typical, and electrical characteristics between ATmega328 variants and ATmega328PB, refer to the specific device datasheets.