Oracle startup error (could not open parameter file)
I see two things wrong here.  Oracle tries to open a parameter file in the $ORACLE_HOME/dbs directory with the format "spfile" + $ORACLE_SID + ".ora".  If it cannot find it, it then tries to open "init" + $ORACLE_SID + ".ora".  This is where the problem(s) are.

By virtue of the fact that Oracle is trying to open a parameter file named "initXE.ora", your SID must be (or was at one point) equal to "XE".

1) This is the easy part, so try this first.  Since (in your comments) doing an echo $ORACLE_SID yields nothing, try setting it.

export ORACLE_SID=XE


Then try starting Oracle again.

2) In your comments above, you show the file is named "initxe.ora".  Is that really the name of the file?  Or did your paste into a comment somehow lowercase the output of the ls command?

The reason I point this out, is that Ubuntu (Linux) has a case-sensitive filesystem.  So initxe.ora and initXE.ora are actually two different files.  So if your comment is correct and the file is lower-cased, you'll want to set your ORACLE_SID to "xe" instead:

export ORACLE_SID=xe


Try that, and see if it works.