How can I partially read a huge CSV file?

Use chunksize:

for df in pd.read_csv('matrix.txt',sep=',', header = None, chunksize=1):
    #do something

To answer your second part do this:

df = pd.read_csv('matrix.txt',sep=',', header = None, skiprows=1000, chunksize=1000)

This will skip the first 1000 rows and then only read the next 1000 rows giving you rows 1000-2000, unclear if you require the end points to be included or not but you can fiddle the numbers to get what you want.