Are there any functional differences between xc16 and C30?
The XC16 toolchain is the upgrade for the C30 toolchain. The C24 and dsPIC toolchains are subsets of the C30. Have a look here
The difference between xc16 and C30 are quiet small.
Maybe your controller is not supported by the C30 compiler.
For a download of old compilers have a look here. The C30 is renamed to PIC C Compiler for PIC24.