Reading the 18F452 Datasheet, RC6 (USART TX) and RC7 (USART RX) are Schmitt Trigger buffered inputs. Also reading the Datasheet (pg. 265 DC Characteristics) shows the following for ST type I/O pins:
Input Logic Low: Min = VSS, Max = .2 VDD
Input Logic High: Min = .8 VDD, Max = VDD

If you are running your PIC at 5 Vdc then that would translate to:
Input Logic Low: Min = 0V, Max = 1V
Input Logic High: Min = 4V, Max 5V

The HC06 running at 3.3V will not output voltage levels that high so the PIC will not recognize a High Logic Level.

You'll need to run the PIC at 3.3V, if you can, or you'll have to level shift the I/O between the two devices.