Source: via Flickr.

With the stock market in nonstop rally mode over the past five years, an investor doesn't need to look far to uncover an abundance of growth stocks. But not all growth stocks are created equal. While some could still lead investors to extraordinary gains, others appear considerably overvalued and could wind up burdening investors with hefty losses.

What exactly is a growth stock? Though it's arbitrary, I'll define a growth stock as any company forecast to grow profits by 10% per year or more over the next five years. To decide what's "cheap," I'll be using the PEG ratio, which compares a company's price-to-earnings ratio to its future growth rate. Any figure around or below one signals a cheap stock.

Here are three companies that fit the bill.

1. Bofi Holding (NYSE:AX)
The easiest way to find high-growth stocks is to look for trendsetters. Bofi Holdings, the parent of the Bank of Internet, is a trendsetter.

Bofi is an Internet-based banking company, providing consumer and commercial banking needs over the Internet. In today's society, everything is about convenience, and Bofi is showing through its quarterly results just how important convenience can be in the banking industry.

In its first-quarter earnings results, released in November, Bofi delivered 41% year-over-year EPS growth to $1.20 per share, 54% core earnings growth, and 60% growth in net interest income. Perhaps even more importantly, total assets grew by 47% to $4.8 billion, its loan portfolio expanded by 63%, and asset quality remained high, with nonperforming loans totaling a reasonable 0.62%. 

How is Bofi Holdings able to grow so quickly? First, it has very little overhead. This isn't an institution that will open up hundreds of branches around the U.S. With the exception of its nearly 400 employees, Bofi's expenses are likely to be lower than many, if not all, of its peers', giving it a margin advantage that its peers can't match.

Source: Bofi Holding.

Secondly, as I alluded to above, it provides the convenience that consumers are looking for, which attracts deposits. As my Foolish colleague Jordan Wathen pointed out in November when Bofi reported its quarterly results, the company pays 0.68% to 0.73% interest on its checking accounts and 1.71% for its average CD. In an environment where lending rates have been near record lows for years, that's an attractive lure for consumers' cash.

The Federal Reserve is widely expected to begin boosting lending rates later this year, so it's possible things could get even better for Bofi in terms of net interest income. Given the stock's PEG ratio well below one and forward P/E of just 12, I'd suggest pointing and clicking your way to a deeper dive on this cheap growth stock.

2. JA Solar Holdings (NASDAQ:JASO)
Who turned the lights on? It very well could be JA Solar, the next cheap growth stock I'd suggest you watch closely.

Source: First Solar.

Like most China-based solar stocks, JA Solar is carrying more debt than cash on hand -- $364 million in net debt, to be exact. This debt was nearly a killer for China's solar industry, as American rivals like First Solar continue to develop comparably more efficient solar panels, allowing them to secure mammoth global deals. Furthermore, Chinese solar companies struggled in the U.S. and Europe because of anti-dumping laws (e.g., tariffs), which made their solar products not nearly as cheap as they first appeared.

The good news, however, is that China's government has gone to bat for its solar industry, making a commitment to install 35 GW of solar production by the end of 2015. Because China's GDP growth is robust, the government can continue to fuel infrastructure-boosting programs, including orders to its solar industry.

Further, we're witnessing strong growth in solar orders from regions outside of the Americas and Europe, where competition has made the going rough for solar module manufacturers. For example, JA Solar is hopeful it will gain orders from India's plans to boost solar production to 100 GW by 2022, and it recently delivered 100 MW of solar modules to a firm in Pakistan.

As of JA Solar's third-quarter earnings results, China and the Asia-Pacific region accounted for 81.2% of its total revenue, up from 78% last year. This is critical for JA Solar, as this region can outpace the growth currently seen in the U.S. and Europe.

Following a quarter in which module shipments grew by 127% year over year, I can't help but be intrigued by JA Solar. As icing on the cake, the company is trading at less than six times forward earnings and a PEG ratio of less than one. 

3. Allegiant Travel (NASDAQ:ALGT)
Lastly, if you want a cheap growth stock with the opportunity to fly high, then you need to give serious consideration to low-cost airline Allegiant Travel.

As with most airlines, Allegiant's biggest concern is fuel costs. The good news here is that crude oil prices are tanking. Although this means airlines will lose out on any near-term oil hedges, it works in their favor over the long term. As airlines see their biggest expense falling, they'll see a potentially sizable boost in their margins.

Source: Flickr user Josh Beasley.

Allegiant, however, isn't your standard airline. Allegiant focuses less on brand names and frills and instead uses low ticket prices to attract fliers. By undercutting the major and national airlines with its ticket prices and then charging customers for the options they choose -- such as carry-on bags, food and drinks, and even seat selection -- the company has developed a way of personalizing the process while also adding high-margin service fees directly to its bottom line.

Another factor working in Allegiant's favor is its size. Because it's choosey about the markets it operates in, and it tends to operate in routes that the majors may ignore, Allegiant has the ability to shut down routes and start up new routes with relative ease.

It's also worth noting that Allegiant's upfront expenses are often lower than those of its peers, as it usually relies on older aircraft. Instead of paying $200 million or more for a new plane, Allegiant buys moderately aged planes or lease and avoids the huge upfront expense of new aircraft.

With its EPS expected to more than double through 2017 and its PEG ratio still reasonable at just 1.17, Allegiant stock could have plenty of upside.