Actually, extrememly different. Parallels runs in a virtual shell (you can google on how VMware works for good technical info), which means that the USB (serial) support not only runs through the drivers and HAL on WindowsXP, it then passes through the USB driver (uhci/ehci module code) on the local OS X system, which includes the Parallels shim to allow that to happen as well.
Bootcamp is just raw booting to Windows, which means (other than the naked Apple hardware) your OS is talking directly in the manner it was designed - through the driver(s), across the HAL and into the chips. Parallels is talking to the generic driver (the virtual shell presents abstracted hardware components to the OS), though the HAL, into the Parallels USB support code, out to the ehci/uhci module code, through the kernel then finally into the chips.