Using a Non-Inverting Buffer for Level Shifting
Use a voltage divider from Arduino Tx to Pi Rx like this. You can connect the Pi Tx to Arduino Rx directly.
Important Update: Connect a similar voltage divider on the Pi Tx - Arduino Rx line also. This is necessary to protect Pi in case of accidental 5 V on the Arduino Rx pin (due to bug in code etc.). I have updated the circuit.



simulate this circuit &ndash; Schematic created using CircuitLab