* Private equity house has 20-year track record in Germany (Adds sources, further background on buyout trend, media deals): FRANKFURT (Reuters) - Axel Springer’s share price jumped more than 20% on Thursday after the German publisher said its main owners were in talks with U.S. investor KKR to take the $5.4 billion company private and pursue a long-term growth strategy. Springer’s shares had lost more than a quarter in value over the past 12 months as investors grew impatient over its heavy digital investments and the drag from circulation declines at legacy media titles including the Bild daily.