No HMC = No VIO.
You could have IVM though.
If you have a p5 with just one install of AIX (no vio and no ivm) then the serial ports (which are owned by the service processor) are passed on to the AIX OS, it knows to do this if the display shows HMC=0, I think as vty rather than tty as on power4, ports.
You cannot use smitty tty to configure them because they are not tty ports, but you can configure them (speed, parity, etc.) from ASMI and they can be used for a console in AIX.
If you get no console output during or after boot, boot to SMS and select the console, then do a service/maintenance mode boot, and again, configure the console to any available vty type device, then do a normal boot and you should be OK.
If you have not installed AIX yet then boot to SMS, select the console and then install AIX and you should be ok.
As said before the default console settings are 19200,8,n,1 but you can set this from ASMI for both the ASMI and the AIX connection.