From the Oxford Corpus of English:
For 30 years, it had gone remorselessly ahead, at about 80% per annum.
An Elan bond, where the bondholder can exercise the right to be repaid in 18 months, currently yields 19 per cent per annum.
With this fresh impetus, the total edible oil processing capacity, including vanaspati, is expected to cross 20 million tonne per annum.
We can currently get about 5% per year from investing in long-dated gilts, so we might aim to get 6% per year from the property.
Penetration of digital has hit 20 % in a year, with incremental revenue per subscriber at $22 per year and growing, he said.
This still leaves teachers $3,000 per year short of accountants, $17,000 short of computer systems analysts, and $25,000 short of engineers.
These example sentences are representative of the most common uses of these two phrases and, as one can see, there is no real difference between per annum and per year in usage.
As kiamlaluno says, per annum is traditionally used more in financial contexts than per year, but these sentences show that per year is also perfectly acceptable.